main: Added entry point for glTransformFeedbackBufferBase

main: Added entry point for glTransformFeedbackBufferBase

v2: Review from Laura Ekstrand
- give more helpful error messages
- factor the lookup code for the xfb and objBuf
- replace some already-existing tabs with spaces
- add comments to explain the cases where xfb == 0 or buffer == 0
- fix the condition for binding the transform buffer or not

v3: Review from Laura Ekstrand
- rename _mesa_lookup_bufferobj_err to
  _mesa_lookup_transform_feedback_bufferobj_err and make it static to avoid a
  future conflict
- make _mesa_lookup_transform_feedback_object_err static

v4: Review from Laura Ekstrand
- add the pdf page number when quoting the spec
- rename some of the symbols to follow the public/private conventions

v5: Review from Laura Ekstrand
- properly rename some of the symbols to follow the public/private conventions
- fix some alignments
- add quotes around a spec citation
- add back a newline I accidentally deleted
- add spaces around the ternary operator usages

Signed-off-by: Martin Peres <martin.peres@linux.intel.com>
Reviewed-by: Laura Ekstrand <laura@jlekstrand.net>