glsl: store number of explicit uniform loactions in gl_shader_program

glsl: store number of explicit uniform loactions in gl_shader_program

This allows us to cleanup the functions that pass this count around,
but more importantly we will be able to call the uniform linking
functions from that backends linker without having to pass this
information to the backend directly via Driver.LinkShader().

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>