compositor: factor out RGB shader.

Simplify RGB shader code and split off common code that could be reused.
This is preparatory work for YUV shaders.

Signed-off-by: Gwenole Beauchesne <gwenole.beauchesne@intel.com>
diff --git a/src/compositor-wayland.c b/src/compositor-wayland.c
index 9df7228..c2a84c4 100644
--- a/src/compositor-wayland.c
+++ b/src/compositor-wayland.c
@@ -177,7 +177,7 @@
 {
 	struct wayland_compositor *c =
 		(struct wayland_compositor *) output->base.compositor;
-	struct weston_shader *shader = &c->base.texture_shader;
+	struct weston_shader *shader = &c->base.texture_shader_rgba;
 	GLfloat *v;
 	int n;