glsl: add std430 interface packing support to ssbo related operations

glsl: add std430 interface packing support to ssbo related operations

v2:
- Get interface packing information from interface's type, not the
  variable type.
- Simplify is_std430 condition in emit_access() for readability (Jordan)
- Add a commment explaing why array of three-component vector case is
  different in std430 than the rest of cases.
- Add calls to std430_array_stride().

v3:
- Simplify size_mul change for std430's case (Jordan)
- Fix commit log lines length (Jordan)
- Pass 'packing' instead of 'is_std430' to emit_access() (Kristian)

Signed-off-by: Samuel Iglesias Gonsalvez <siglesias@igalia.com>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>
Reviewed-by: Kristian Høgsberg <krh@bitplanet.net>