Fix for running OpenGL ES2 with SDL on desktop

[realtradam]

System: Artix Linux, Desktop PC

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When trying to compile Raylib configured with SDL and OpenGL ES2 it will fail like the following:

By editing rcore_desktop_sdl.c to not include SDL_opengl.h when compiling for OpenGL ES2 it compiles and works. See here:

However when looking through the headerfiles of SDL I see there is a SDL_opengles2.h which I suspect should be used in place of SDL_opengl.h but it fails to compile when I do that. I am not entire sure why, I tried to do some digging but havent come up with anything yet, if anyone else has any insight it would be much appreciated however I think the current fix is sufficient.

I have not tested this yet on Windows or Mac. I can test it on Windows and Mac most likely sometime later this week or next week.

[raysan5]

@realtradam thanks for the review! Afaik, OpenGL ES 2.0 could be supported in different ways:

• On desktop, using the specific OpenGL extension for ES
• On ES-specific platforms, directly using OpenGL ES

Probably this implementation could require some more work in the future...