glsl: Add std430 related member functions to glsl_type class

glsl: Add std430 related member functions to glsl_type class

They are used to calculate size, base alignment and array stride values
for a glsl_type following std430 rules.

v2:
- Paste OpenGL 4.3 spec wording as it mentions stride of array. (Jordan)

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>