#11371 Remove GLEW dependency and use Qt's QOpenGLFunctions

[kriben]

Replace embedded GLEW library with Qt's native OpenGL function loading (QOpenGLFunctions/QOpenGLExtraFunctions) to resolve Qt 6 compatibility warnings about GLEW conflicts with qopenglfunctions.h.

• Update CVF_CALLSITE_OPENGL macro to provide QOpenGLExtraFunctions pointer
• Replace GLEW initialization with Qt OpenGL context validation
• Keep direct calls for fixed-function GL (glBegin, glEnd, glMatrixMode, etc.)
• Remove glew/ directory containing embedded GLEW source

[kriben]

Hi @sigurdp!

@magnesj and I would like your input on this PR.

Qt6 provides its own OpenGL function resolution infrastructure that replaces the need for GLEW.
Here's what Qt6 offers instead:

1. QOpenGLFunctions — This class resolves OpenGL ES 2.0 (and desktop GL equivalent) functions portably. You inherit from it or access it via QOpenGLContext::functions(). It handles all the platform-specific function pointer loading that GLEW traditionally did.
2. QOpenGLExtraFunctions — Extends the above with OpenGL ES 3.0/3.1/3.2 functions (and their desktop equivalents), available via QOpenGLContext::extraFunctions().

Pros:

• GLEW manages its own global state and context assumptions, which can clash with Qt's context management (especially with multiple contexts or context sharing).
• Qt's wrappers are context-aware and integrate cleanly with QOpenGLContext, QOpenGLWidget, and the rest of the Qt rendering pipeline.
• We eliminate an external dependency for something Qt already handles well.

Cons:

• Even more dependent on Qt.

[kriben]

[1022/2965] Building CXX object Fwk/AppFwk/cafViewer/CMakeFiles/cafViewer.dir/cafViewer.cpp.o
Warning that started the investigation:

In file included from /usr/include/x86_64-linux-gnu/qt5/QtGui/QOpenGLContext:1,
                 from /home/resinsight/projects/ResInsight/Fwk/AppFwk/cafViewer/cafViewer.cpp:75:
/usr/include/x86_64-linux-gnu/qt5/QtGui/qopenglcontext.h:55:2: warning: #warning qopenglfunctions.h is not compatible with GLEW, GLEW defines will be undefined [-Wcpp]
   55 | #warning qopenglfunctions.h is not compatible with GLEW, GLEW defines will be undefined
      |  ^~~~~~~
/usr/include/x86_64-linux-gnu/qt5/QtGui/qopenglcontext.h:56:2: warning: #warning To use GLEW with Qt, do not include <qopengl.h> or <QOpenGLFunctions> after glew.h [-Wcpp]
   56 | #warning To use GLEW with Qt, do not include <qopengl.h> or <QOpenGLFunctions> after glew.h
      |  ^~~~~~~

[magnesj]

Example from build using Qt 6.7
https://github.com/OPM/ResInsight/actions/runs/22141649724/job/64007192477#step:23:1203

[magnesj]

Here are 4 relevant Qt documentation links for this PR:

1. [QOpenGLFunctions](https://doc.qt.io/qt-6/qopenglfunctions.html) — Cross-platform OpenGL ES 2.0 function resolution
2. [QOpenGLExtraFunctions](https://doc.qt.io/qt-6/qopenglextrafunctions.html) — ES 3.x extensions used in the CVF_CALLSITE_OPENGL macro
3. [Changes to Qt OpenGL (Qt 6)](https://doc.qt.io/qt-6/opengl-changes-qt6.html) — Qt 6 OpenGL module restructuring and porting notes
4. [QOpenGLWidget](https://doc.qt.io/qt-6/qopenglwidget.html) — Widget base class used throughout ResInsight