texturator: Use correct GL extension header

gl2ext.h is the extenstion 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.

[1]: https://www.khronos.org/registry/OpenGL/index_es.php#headers
2 jobs for gles_header_fix in 1 minute and 20 seconds (queued for 1 second)