Simplify add reference image size implementation and fix segfaulting tests

Previously, there was a dance between invoke, execute, and draw. This
can probably be resolved, but is a high-risk for undo bugs. This
simplifies the logic flow to just a traditional _invoke -> _execute.

I add a new feature test to at least make sure it does something, and
this also fixes the segfault in tool tests as it no longer requires the
launching of the file browser.

src/bonsai/test/tool/test_drawing.py

@@ -938,7 +938,7 @@ class TestAddReferenceImage(NewFile):
 
         obj = bpy.data.objects["IfcAnnotation/image"]
         assert obj is not None
-        assert tool.Cad.are_vectors_equal(obj.dimensions, Vector((3.53982, 2.0, 0.0)))
+        assert tool.Cad.are_vectors_equal(obj.dimensions, Vector((1.0, 0.565, 0.0)))
 
         material = obj.active_material
         assert material