Fix missing PLATFORM_DESKTOP_SDL checks.

[2Bear]

There are two missing PLATFORM_DESKTOP_SDL checks in rlgl.h, so that OpenGL ES support in PLATFORM_DESKTOP_SDL is broken.

Add missing PLATFORM* checks can fix that, but a bit dirty.

BTW, consider that more platforms will be supported here, so maybe rlgl.h should be more independent.

[raysan5]

@2Bear thanks for the review!

BTW, consider that more platforms will be supported here, so maybe rlgl.h should be more independent.

raylib is intended for OpenGL so rlgl should already cover all platforms supporting the technology properly. OpenGL ES support on desktop platforms is not a common use case, they should target OpenGL desktop instead.