Codechange: [OpenGL] Load all OpenGL functions dynamically.

[michicc]

Motivation / Problem

Some Linux OpenGL drivers don't use the global system dispatch table.

Description

Link all OpenGL functions dynamically to make sure we use the right one.

Limitations

OpenGL headers are not uniform among systems, which might cause compile problems on more obscure systems.

Checklist for review

Some things are not automated, and forgotten often. This list is a reminder for the reviewers.

• The bug fix is important enough to be backported? (label: 'backport requested')
• This PR affects the save game format? (label 'savegame upgrade')
• This PR affects the GS/AI API? (label 'needs review: Script API')
  • ai_changelog.hpp, gs_changelog.hpp need updating.
  • The compatibility wrappers (compat_*.nut) need updating.
• This PR affects the NewGRF API? (label 'needs review: NewGRF')
  • newgrf_debug_data.h may need updating.
  • PR must be added to API tracker

[Milek7]

Alternative approach for consideration, use autogenerated loader instead: Milek7@127aaa9

As system OpenGL headers are not used at all, I removed library searching and linking from CMake files.
For win32 it also needs win32_v.cpp from this PR.

[TrueBrain]

With this PR, I can remove link_package(OpenGL TARGET OpenGL::GL) from CMakeLists.txt. This has the benefit that our generic linux binaries are a lot smaller again, as it no longer drags in OpenGL.so and because of that, X11 and friends. That won't really be a generic linux binary.

Without the link_package, SDL takes care of all the dynamic loading, removing those dependencies from the binary.

I can PR that after this PR, if you like, but maybe the link_package is also not needed on other targets now?

[michicc]

It is still needed for Win32 because of the wgl calls, but opengl32.dll is a system DLL everywhere, so no problem.
OSX I'd have to check.

[frosch123]

works for me