GLES3: Split EGL includes in `platform_egl.h`

EGL brings in platform-specific headers such as the dreaded `windows.h`,
and `platform_gl.h` is used throughout `drivers/gles3` for basic OpenGL
types such as `GLuint`. We don't want `windows.h` pollution there.

Note for Android: EGL seems used explicitly only via `rasterizer_gles3.cpp`
to enable GL debug printing, and some custom stuff in `config.cpp`.