ifcviewer-full: length tool (2/3/4+ point distance, angle, polygon area)

ViewportWindow trades the area_tool_active_ bool for an enum ToolMode
{None, Area, Length}; the existing surfacePickedInTool signal carries
both, the app dispatches on toolMode().  Esc exits any active tool;
Backspace/Delete in length mode emits toolBackspacePressed which the
length tool uses to remove the last point.

LengthMeasurement collects clicked world-space points and adapts the
readout: 2pt → distance + axis-aligned ΔX/ΔY/ΔZ, 3pt → angle at the
middle vertex + triangle area, 4+pt → best-fit-plane PCA + shoelace
when planar (RMS plane distance / bbox diag < 1e-3) else fan
triangulation, with the chosen method labelled in the readout.  Per-
segment lengths float at each midpoint.

OverlayRenderer grows three new pipelines to support this:
  - point sprite shader: gl_PointCoord-based outlined disc with
    fwidth-smoothed inner/stroke bands, a single draw call.
  - line shader: CPU-expand each segment to 6 verts carrying both
    endpoints + (side, along) corner index; vertex shader computes
    the screen-space perpendicular and offsets accordingly.  Real
    outlined lines independent of the driver's glLineWidth clamp.
  - screen-space rect shader: HUD + label backgrounds drawn as raw
    GL quads in NDC.  QPainter::fillRect on QOpenGLPaintDevice was
    silently dropping fills across drivers; bypassing it entirely
    via this shader makes backgrounds reliable.  Cull-face is also
    explicitly disabled here — GL_TRIANGLES respects it but the
    line/point primitives don't, so this was the one path needing
    the fix.

setOverlayLines / setOverlayPoints take an inner color, an outline
color, and an extra-pixels-per-side stroke amount.  Lines + points
draw with GL_ALWAYS so measurement annotations stay visible through
geometry; highlight tris stay depth-aware (GL_LEQUAL) so area
shading still tints the surface in place.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/ifcviewer/ViewportWindow.cpp

@@ -1690,12 +1690,19 @@ void ViewportWindow::keyPressEvent(QKeyEvent* event) {
             return;
         }
     }
-    // Esc also exits the area tool.
-    if (area_tool_active_
-        && key == Qt::Key_Escape
-        && !event->isAutoRepeat()) {
-        toggleAreaTool();
-        return;
+    // Esc exits any active measurement tool.  Backspace/Delete in length
+    // mode removes the last point — emitted as a signal because the
+    // library doesn't track per-tool semantic state.
+    if (tool_mode_ != ToolMode::None && !event->isAutoRepeat()) {
+        if (key == Qt::Key_Escape) {
+            setToolMode(ToolMode::None);
+            return;
+        }
+        if (tool_mode_ == ToolMode::Length
+            && (key == Qt::Key_Backspace || key == Qt::Key_Delete)) {
+            emit toolBackspacePressed();
+            return;
+        }
     }
     QWindow::keyPressEvent(event);
 }
@@ -3492,7 +3499,7 @@ void ViewportWindow::handleMouseRelease(QMouseEvent* e) {
     if (active_button_ == Qt::LeftButton
         && !section_tool_active_
         && (e->pos() - last_mouse_pos_).manhattanLength() < 5) {
-        if (area_tool_active_) {
+        if (tool_mode_ != ToolMode::None) {
             emit surfacePickedInTool(e->pos().x(), e->pos().y(),
                                      int(e->modifiers()));
         } else {
@@ -3868,9 +3875,18 @@ bool ViewportWindow::pickMeshLocalAt(int x, int y, MeshLocalPick& out) {
     return false;
 }
 
+void ViewportWindow::setToolMode(ToolMode mode) {
+    if (tool_mode_ == mode) return;
+    tool_mode_ = mode;
+    emit toolModeChanged(mode);
+}
+
 void ViewportWindow::toggleAreaTool() {
-    area_tool_active_ = !area_tool_active_;
-    emit areaToolToggled(area_tool_active_);
+    setToolMode(tool_mode_ == ToolMode::Area ? ToolMode::None : ToolMode::Area);
+}
+
+void ViewportWindow::toggleLengthTool() {
+    setToolMode(tool_mode_ == ToolMode::Length ? ToolMode::None : ToolMode::Length);
 }
 
 void ViewportWindow::setHighlightTriangles(const std::vector<float>& world_xyz,
@@ -3881,6 +3897,35 @@ void ViewportWindow::setHighlightTriangles(const std::vector<float>& world_xyz,
     requestUpdate();
 }
 
+void ViewportWindow::setOverlayLines(const std::vector<float>& world_xyz,
+                                      float r, float g, float b, float a,
+                                      float line_width,
+                                      float sr, float sg, float sb, float sa,
+                                      float stroke_extra) {
+    if (!gl_initialized_) return;
+    context_->makeCurrent(this);
+    overlay_renderer_.setOverlayLines(world_xyz, r, g, b, a, line_width,
+                                      sr, sg, sb, sa, stroke_extra);
+    requestUpdate();
+}
+
+void ViewportWindow::setOverlayPoints(const std::vector<float>& world_xyz,
+                                       float r, float g, float b, float a,
+                                       float pixel_size,
+                                       float sr, float sg, float sb, float sa,
+                                       float stroke_extra) {
+    if (!gl_initialized_) return;
+    context_->makeCurrent(this);
+    overlay_renderer_.setOverlayPoints(world_xyz, r, g, b, a, pixel_size,
+                                       sr, sg, sb, sa, stroke_extra);
+    requestUpdate();
+}
+
+void ViewportWindow::setOverlayLabels(const std::vector<OverlayRenderer::Label>& labels) {
+    overlay_renderer_.setOverlayLabels(labels);
+    requestUpdate();
+}
+
 void ViewportWindow::setHudText(const QString& text) {
     overlay_renderer_.setHudText(text);
     requestUpdate();