Fix the reference to GLES3/gl3ext.h to instead point to GLES2/gl2ext.h - the former is only provided for backwards compatibility, while the latter is the correct header.
Fix the reference to GLES3/gl3ext.h to instead point to GLES2/gl2ext.h - the former is only provided for backwards compatibility, while the latter is the correct header.

See https://www.khronos.org/registry/OpenGL/index_es.php

grimfang4/sdl-gpu,ademuri/sdl-gpu,grimfang4/sdl-gpu,grimfang4/sdl-gpu,ademuri/sdl-gpu,ademuri/sdl-gpu,grimfang4/sdl-gpu,ademuri/sdl-gpu