Fix #1645 (needs #1646 , #1644 , #1650 )

TL;DR

OpenGL code in the core is now enclosed in the new module Sofa.GL.
If you need to call OpenGL code, you will need to find_package() it and add it to target_link_library().

Description

OpenGL is spread a bit everywhere in the core, and it is difficult to bypass it if you dont need it (typically for batch mode or headless system).
The macro SOFA_NO_OPENGL is a really Quick and Dirty solution to this problem.
For example, people include sofa/helper/system/gl.h (or any gl-related header), even without needing OpenGL actually.
And in this header, SOFA_NO_OPENGL does include (or not) the good gl.h header according to your OS. So if you enable this header, you still include gl.h, which does nothing.
This is quite nonsensical.
Moreover, if the macro is not well handled in CMake, you may have some weird things happening.

Solution

This PR regroups all OpenGL related code in the core in a single package, needing only SofaHelper and SofaDefaulttype.
If you wish to be able to call OpenGL things, just find_package() it and link it to your library

find_package(Sofa.GL REQUIRED)
...
target_add_library(${Target} PUBLIC .... Sofa.GL)

You can even conditionnally test if OpenGL is present or not with

if(Sofa.GL_FOUND)
...
endif()

if needed (to compile specific components in your module for example).

And even more, if you want to compile specific parts of your code with OpenGL (not a good idea but still), you can use this like an external dependency.
Example with ColourPickingVisitor in SofaGuiCommon.
CMakeLists.txt:

sofa_find_package(Sofa.GL) # create the cmake var SOFAGUICOMMON.GL_HAVE_SOFA.GL

config.h.in

#define SOFAGUICOMMON_HAVE_SOFA_GL @SOFAGUICOMMON_HAVE_SOFA.GL@

ColourPickingVisitor.cpp

void ColourPickingVisitor::processTriangleModel(simulation::Node * node, sofa::component::collision::TriangleCollisionModel<sofa::defaulttype::Vec3Types> * tmodel)
{
#ifdef SOFAGUICOMMON_HAVE_SOFA_GL
    using namespace sofa::core::collision;
    using namespace sofa::defaulttype;
    glDisable(GL_LIGHTING);
    ....
#endif // SOFAGUICOMMON_HAVE_SOFA_GL

Compatibility with actual code and breaking things

This new module follows the new targeted architecture for Sofa(NG), hence the Sofa.GL syntax.
All classes/functions are now in the namespace sofa::gl and in the folder sofa/gl; e.g Texture,
sofa/gl/Texture.h

namespace sofa::gl
{
	class Texture {...}
}

To avoid breaking (a lot of) code, a compatibility layer has been implemented, so the existing code still works:

#include <sofa/helper/gl/Texture.h
...
sofa::helper::gl::Texture = ....;

and will either:

• create an error if you did not link against Sofa.GL
• generate a deprecation warning if you did link but still using old paths.
  So for 99.999% of code, all you have to do is to link against Sofa.GL (or even nothing if you code is already linking against modules linking now against Sofa.GL like SofaOpenGlVisual)

Real breaking code

ONE function has been removed from core::visual::VisualParam :
helper::gl::Framebuffer getFramebuffer()
which was used to get the current bound framebuffer in OpenGL (getting a Sofa utility class Framebuffer).
Two reasons why it has been removed:

• nobody uses it in all the repository
• one can just use a real OpenGL function to get the bound framebuffer, with

glGetIntegerv(GL_FRAMEBUFFER_BINDING, &result);

for example.

Notes

The compilation of Sofa.GL is itself enabled with the CMake variable SOFA_NO_OPENGL (weird name but I wanted to keep the same "reasoning" for now)
Use of SOFA_NO_OPENGL macro in the code has been removed from the Sofa codebase only, not the plugins (in a future PR)

😅

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Reviewers will merge this pull-request only if

• it builds with SUCCESS for all platforms on the CI.
• it does not generate new warnings.
• it does not generate new unit test failures.
• it does not generate new scene test failures.
• it does not break API compatibility.
• it is more than 1 week old (or has fast-merge label).