mesa: rename MaxTransformFeedbackSeparateAttribs to MaxTransformFeedbackBuffers

mesa: rename MaxTransformFeedbackSeparateAttribs to MaxTransformFeedbackBuffers

This is a cleanup for ARB_transform_feedback3, where
GL_MAX_TRANSFORM_FEEDBACK_BUFFERS is introduced for interleaved attribs and
has the same meaning as GL_MAX_.._SEPARATE_ATTRIBS for separate attribs.

Also, the maximum number of TFB buffers is reduced from 32 to 4, which makes
this patch useful even without the extension.
I don't know of any hardware which can do more than 4.

Reviewed-by: Brian Paul <brianp@vmware.com>