gl-renderer: move functions into gl-shaders.c

These functions are related to shaders, so they are more at home in
gl-shaders.c. gl-renderer.c is too long already.

This allows making a couple functions static while the moved functions
become non-static. Future changes turn some of these functions into
static again, with the ultimate goal of making struct gl_shader opaque.

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.com>