mesa/glsl: move BlendSupport bitfield to gl_program

mesa/glsl: move BlendSupport bitfield to gl_program

This will let us to make _CurrentFragmentProgram a gl_program pointer
allowing for simpilifications to be made.

We also need to add a field to gl_shader to hold it during parsing.

In gl_program we put it inside a union in anticipation of moving
more fields here that can be only fs or vertex stage fields.

Reviewed-by: Eric Anholt <eric@anholt.net>