This patch adds two new fields to the gl_transform_feedback_info
struct:

- BufferStride records the total number of components (per vertex)
  that transform feedback is being instructed to store in each buffer.

- Outputs[i].DstOffset records the offset within the interleaved
  structure of each transform feedback output.

These values are needed by the i965 gen6 and r600g back-ends, so it
seems better to have the linker provide them rather than force each
back-end to compute them independently.

Also, DstOffset helps pave the way for supporting
ARB_transform_feedback3, which allows the transform feedback output to
contain holes between attributes by specifying
gl_SkipComponents{1,2,3,4} as the varying name.

Reviewed-by: Marek Olšák <maraeo@gmail.com>