GPU: minor changes to support standalone GHOST builds

- Move gpuPush/Pop from GPU_draw.h into GPU_state.h
  as this is for pushing/popping state.
- Add 'GPU_STANDALONE' define, to bypass use of user-preferences
  for theme colors and pixelsize, as well as pbvh init/free functions.

Needed to get GHOST tests working again.

source/blender/gpu/intern/gpu_select_sample_query.c

@@ -26,9 +26,8 @@
 
 #include <stdlib.h>
 
-#include "GPU_immediate.h"
-#include "GPU_draw.h"
 #include "GPU_select.h"
+#include "GPU_state.h"
 #include "GPU_glew.h"
 
 #include "MEM_guardedalloc.h"
@@ -37,10 +36,6 @@
 
 #include "BLI_utildefines.h"
 
-#include "PIL_time.h"
-
-#include "BKE_global.h"
-
 #include "gpu_select_private.h"
 
 /* Ad hoc number of queries to allocate to skip doing many glGenQueries */