CodeCleanup: Added enums to opengl render functions

Motivation the functions get 3 different kind of flag parameters (ImBuf,
DrawType, OffscreenRendering) the naming of the flags were not clear,
leading to mistakes and unnecessary time spend debugging.

source/blender/windowmanager/intern/wm_files.c

@@ -105,6 +105,7 @@
 #include "ED_outliner.h"
 #include "ED_screen.h"
 #include "ED_view3d.h"
+#include "ED_view3d_offscreen.h"
 #include "ED_util.h"
 #include "ED_undo.h"