DRW: Add new Draw Manager OpenGL Context.

This separate context allows two things:
- It allows viewports in multi-windows configuration.
- F12 render can use this context in a separate thread and do a non-blocking render.

The downside is that the context cannot be used while rendering so a request to refresh a viewport will lock the UI. This is something that will be adressed in the future.

Under the hood what does that mean:
- Not adding more mess with VAOs management in gawain.
- Doing depth only draw for operators / selection needs to be done in an offscreen buffer.
- The 3D cursor "autodis" operator is still reading the backbuffer so we need to copy the result to it.
- All FBOs needed by the drawmanager must to be created/destroyed with its context active.
- We cannot use batches created for UI in the DRW context and vice-versa. There is a clear separation of resources that enables the use of safe multi-threading.

1 files changed, 1 insertions, 0 deletions

diff --git a/source/blender/windowmanager/wm_window.h b/source/blender/windowmanager/wm_window.h
index 0264955a4c5..652cefb1a54 100644
--- a/source/blender/windowmanager/wm_window.h
+++ b/source/blender/windowmanager/wm_window.h
@@ -58,6 +58,7 @@ void		wm_window_process_events	(const bContext *C);
 void		wm_window_process_events_nosleep(void);
 
 void		wm_window_make_drawable(wmWindowManager *wm, wmWindow *win);
+void		wm_window_reset_drawable(void);
 
 void		wm_window_raise			(wmWindow *win);
 void		wm_window_lower			(wmWindow *win);