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/python/gpu/CMakeLists.txt

@@ -19,7 +19,9 @@ set(INC
   .
   ../../blenkernel
   ../../blenlib
+  ../../editors/include
   ../../gpu
+  ../../imbuf
   ../../makesdna
   ../../../../intern/glew-mx
   ../../../../intern/guardedalloc

source/blender/python/gpu/gpu_py_offscreen.c

@@ -43,7 +43,8 @@
 #include "GPU_framebuffer.h"
 #include "GPU_texture.h"
 
-#include "../editors/include/ED_view3d.h"
+#include "ED_view3d.h"
+#include "ED_view3d_offscreen.h"
 
 #include "../mathutils/mathutils.h"