Also declare the GL functions in tr_local.h so there is compile error
for non-core GL functions instead of SEGFAULT from dereferencing a NULL
pointer.
Disable the non-functional stencil shadow code that hasn't been updated
to use OpenGL 3.2 core compatible drawing.
- Parse OpenGL version in sdl_glimp.c to share with both renderers.
- Add GL_VERSION_ATLEAST(major, minor) macro.
- Get address of glGetStringi if using OpenGL 3.
- Fix glConfig.extensions_string when using GL3 core context in
opengl2 renderer.
- Make opengl1 renderer's gfxinfo support qglGetStringi too.
Get all OpenGL functions using SDL_GL_GetProcAddress(). This makes it
easier to cross-arch compile on Linux and add support for OpenGL ES
in the future.
Users still have to supply their own libSDL2 for cross-arch compiling
on Linux. But now the user does not have to re-install libgl1-mesa-dev
package for i386 or amd64 on Debian when switching between compiling
ioquake3 for x86 and x86_64.
