glsl: Generalize compute_packing_order for varying structs.

glsl: Generalize compute_packing_order for varying structs.

This patch paves the way for allowing varying structs by generalizing
varying_matches::compute_packing_order to handle any type of varying.
Previously, we packed in the order (vec4, vec2, float, vec3), with
matrices being packed according to the size of their columns.  Now, we
pack everything according to its number of components mod 4, in the
order (0, 2, 1, 3).

There is no behavioural change for vectors.  Matrices are now packed
slightly differently:

- mat2x2 gets assigned PACKING_ORDER_VEC4 instead of
  PACKING_ORDER_VEC2.  This is slightly better, because it guarantees
  that the matrix occupies a single varying slot.

- mat2x3 gets assigned PACKING_ORDER_VEC2 instead of
  PACKING_ORDER_VEC3.  This is kind of a wash.  Previously, mat2x3 had
  a 25% chance of having neither of its columns double parked, a 50%
  chance of having exactly one of its columns double parked, and a 25%
  chance of having both of its columns double parked.  Now it always
  has exactly one of its columns double parked.

- mat3x3 gets assigned PACKING_ORDER_SCALAR instead of
  PACKING_ORDER_VEC3.  This doesn't affect much, since in both cases
  there is no guarantee of how the matrix will be aligned.

- mat4x2 gets assigned PACKING_ORDER_VEC4 instead of
  PACKING_ORDER_VEC2.  This is slightly better for the same reason as
  in mat2x2.

- mat4x3 gets assigned PACKING_ORDER_VEC4 instead of
  PACKING_ORDER_VEC3.  This is slightly better for the same reason as
  in mat2x2.

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