mesa: make _CurrentFragmentProgram a gl_program struct pointer

mesa: make _CurrentFragmentProgram a gl_program struct pointer

Making this point to a gl_program struct rather than a gl_shader_program
struct will allow use to later also make the CurrentProgram array hold
gl_program structs which in turn will allow for code simpilifcation.

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