texturator: Use correct GL extension header
gl2ext.h is the extension header for OpenGL ES 2.0 and all later versions according to the Khronos documentation 1. gl3ext.h is either an empty stub, or may not even exist on some platforms.
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gl2ext.h is the extension header for OpenGL ES 2.0 and all later versions according to the Khronos documentation 1. gl3ext.h is either an empty stub, or may not even exist on some platforms.