Gawain: add immRect utility functions (replaces legacy glRect)

Caller is responsible for setting up vertex format, binding a shader program, and setting the color *before* calling immRect.

source/blender/gpu/GPU_immediate.h

@@ -31,6 +31,7 @@
 #pragma once
 
 #include "gawain/immediate.h"
+#include "gawain/imm_util.h"
 #include "GPU_shader.h"
 
 /* Extend immBindProgram to use Blender’s library of built-in shader programs.