glsl/linker: Subdivide the first phase of varying assignment.

glsl/linker: Subdivide the first phase of varying assignment.

This patch further subdivides the loop that assigns varying locations
into two phases: one phase to match up the varyings between shader
stages, and one phase to assign them varying locations.

In between the two phases the matched varyings are stored in a new
data structure called varying_matches.  This will free us to be able
to assign varying locations in any order, which will pave the way for
packing varyings.

Note that the new varying_matches::assign_locations() function returns
the number of varying slots that were used; this return value will be
used in a future patch.

Reviewed-by: Eric Anholt <eric@anholt.net>