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.

main/tests/test_shader_lang.cpp

@@ -342,7 +342,7 @@ MainLoop *test() {
 	Set<String> types;
 	types.insert("spatial");
 
-	Error err = sl.compile(code, dt, rm, types);
+	Error err = sl.compile(code, dt, rm, types, NULL);
 
 	if (err) {