ifcviewer: fix HiZ depth blit and make occlusion test conservative

The HiZ pipeline had two bugs causing false occlusions:

1. The scaling depth blit (glBlitFramebuffer from window-size to HiZ-size)
   produced GL_INVALID_VALUE on some drivers. Replace with a fullscreen-
   triangle shader that samples the resolved depth and writes gl_FragDepth.

2. The resolve texture used GL_DEPTH_COMPONENT24 but Qt's default FBO uses
   D24S8 (depth+stencil). Mismatched formats cause the MSAA resolve blit
   to fail. Fix by using GL_DEPTH24_STENCIL8 for the resolve texture.

Additionally, the occlusion test was too aggressive for scenes with
compressed depth ranges (entire scene in 0.99-1.0). Change from
"max over coarse mip texels" to "reject only if ALL fine-mip texels
agree the AABB is behind them", with early-out on first non-occluding
texel and a 64-sample cap.

Also fix IFC_HIZ_MOTION=0 being treated as enabled (checked env var
existence, not value).

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/ifcviewer/ViewportWindow.h

@@ -332,6 +332,8 @@ private:
     // (hiz_vp_ + hiz_vp_valid_) so next frame's cull can test AABBs
     // against a slightly-stale depth.  Skipped for the pick pass and when
     // IFC_NO_HIZ=1.
+    GLuint hiz_downsample_program_ = 0;
+    GLuint hiz_downsample_vao_ = 0;
     GLuint hiz_fbo_ = 0;
     GLuint hiz_depth_tex_ = 0;
     GLuint hiz_resolve_fbo_ = 0;         // full-size single-sample resolve