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/gawain/imm_util.h

@@ -0,0 +1,18 @@
+
+// Gawain immediate mode drawing utilities
+//
+// This code is part of the Gawain library, with modifications
+// specific to integration with Blender.
+//
+// Copyright 2016 Mike Erwin
+//
+// This Source Code Form is subject to the terms of the Mozilla Public License, v. 2.0. If a copy of
+// the MPL was not distributed with this file, You can obtain one at https://mozilla.org/MPL/2.0/.
+
+#pragma once
+
+
+// Draw 2D rectangles (replaces glRect functions)
+// caller is reponsible for vertex format & shader
+void immRectf(unsigned pos, float x1, float y1, float x2, float y2);
+void immRecti(unsigned pos, int x1, int y1, int x2, int y2);