Cleanup: get rid of last G.main's in bpy area.

Essentially 'validating' them as G_MAIN, and adding some asserts in
dubious places, that handled IDs are actually in G_MAIN.

source/blender/python/intern/gpu_offscreen.c

@@ -36,6 +36,7 @@
 #include "WM_types.h"
 
 #include "BKE_global.h"
+#include "BKE_library.h"
 
 #include "ED_screen.h"
 
@@ -169,7 +170,7 @@ static PyObject *pygpu_offscreen_draw_view3d(BPy_GPUOffScreen *self, PyObject *a
 	MatrixObject *py_mat_modelview, *py_mat_projection;
 	PyObject *py_scene, *py_region, *py_view3d;
 
-	struct Main *bmain = G.main;  /* XXX UGLY! */
+	struct Main *bmain = G_MAIN;  /* XXX UGLY! */
 	Scene *scene;
 	View3D *v3d;
 	ARegion *ar;
@@ -191,6 +192,8 @@ static PyObject *pygpu_offscreen_draw_view3d(BPy_GPUOffScreen *self, PyObject *a
 		return NULL;
 	}
 
+	BLI_assert(BKE_id_is_in_gobal_main(&scene->id));
+
 	fx = GPU_fx_compositor_create();
 
 	fx_settings = v3d->fx_settings;  /* full copy */