specs: Sync framebuffer_flip_y with OpenGL registry
The recently accepted Mesa MR !1002 (merged) was not fully in sync with the respective OpenGL registry PR [0] before it was merged. This commit fixes this, by making the following amendments:
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sync framebuffer_flip_y extension text to what has been merged upstream to the GL spec master branch (i.e. now with proper extension # for the GL core part)
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update generated glcorearb.h to stay in sync with upstream GL registry
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extend glext.h to match changes made in upstream GL registry (without removing the Mesa-specific guards that do not exist in the GL registry version)
[0] https://github.com/KhronosGroup/OpenGL-Registry/pull/277/files