This cl adds a utility function that assigns the correct buffer
for each linked transform feedback variable. All the information
about location and size is sent to the VertexProcessor for use
during rendering.

Change-Id: I942805250804f56805de1fc117024c20976e83a0
Reviewed-on: https://swiftshader-review.googlesource.com/5174Tested-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <capn@google.com>