glsl: Set up uniform initializers by walking the shaders after linking.

glsl: Set up uniform initializers by walking the shaders after linking.

Previously, uniform initializers were handled by ir_to_mesa as it made
its Parameters list.  However, uniform values are global to all
shaders, and the value set in one Parameters list wasn't propagated to
the other gl_program->Parameters lists.  By going back through the
general Mesa uniform handling, we make sure that all gl_programs get
updated values, and also successfully separate uniform initializer
handling from ir_to_mesa gl_program generation.

Fixes:
glsl-uniform-initializer-5.