Implement global and per instance shader uniforms.

Adds two keywords to shader language for uniforms:
-'global'
-'instance'

This allows them to reference values outside the material.

scene/scene_string_names.h

@@ -207,6 +207,8 @@ public:
 	StringName window_input;
 
 	StringName theme_changed;
+	StringName shader_overrides_group;
+	StringName shader_overrides_group_active;
 
 	enum {
 		MAX_MATERIALS = 32