Rajdeep Singh b8cfd196f4 Fix CGAL::draw() black rendering on OpenGL < 4.3 (issue #6754) ...

Fix compatibility shaders and OpenGL version detection that caused
CGAL::draw() to render solid black geometry on systems with OpenGL < 4.3.

Basic_shaders.h:
- VERTEX_SOURCE_COLOR_COMP: change 'varying' to 'attribute' for vertex
  inputs (a_Pos, a_Normal, a_Color) as required by GLSL 1.10/1.20
- VERTEX_SOURCE_COLOR_COMP: replace undefined 'mv_matrix' reference with
  'mat3(u_Mv) * a_Normal' matching the declared uniform
- VERTEX_SOURCE_P_L_COMP: change 'varying' to 'attribute' for vertex
  inputs (a_Pos, a_Color)

qglviewer_impl.h:
- Initialize is_ogl_4_3 to false in defaultConstructor() to avoid UB
- Fix version check: use major < 4 || (major == 4 && minor < 3) instead
  of major != 4, so OpenGL 4.0-4.2 correctly falls back and 5+ works

Basic_viewer.h:
- Add std::cerr warning when compatibility shaders are selected

2026-03-24 15:50:58 +05:30

..

CGAL

Fix CGAL::draw() black rendering on OpenGL < 4.3 (issue #6754)

2026-03-24 15:50:58 +05:30