glsl: Update lower_packed_varyings to handle varying structs.

glsl: Update lower_packed_varyings to handle varying structs.

This patch adds code to lower_packed_varyings to handle varyings of
type struct.  Varying structs are currently packed in the most naive
possible way (in declaration order, with no gaps), so there is a
potential loss of runtime efficiency.  In a later patch it would be
nice to replace this with a "flattening" approach (wherein a varying
struct is flattened to individual varyings corresponding to each of
its structure elements), so that the linker can align each structure
element independently.  However, that would require a significantly
more complex implementation.

Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>
Reviewed-by: Eric Anholt <eric@anholt.net>