Experimental feature to lock the interface while rendering

Added function called WM_set_locked_interface which does
two things:

- Prevents event queue from being handled, so no operators
  or values are even possible to run or change. This prevents
  any kind of "destructive" action performed from user while
  rendering.

- Locks interface refresh for regions which does have lock
  set to truth in their template. Currently it's just a 3D
  viewport, but in the future more regions could be considered
  unsafe, or we could want to lock different parts of
  interface when doing different jobs.

  This is needed because 3D viewport could be using or changing
  the same data as renderer currently uses, leading to threading
  conflict.

Notifiers are still allowed to handle, so render progress is
seen on the screen, but would need to doublecheck on this, in
terms some notifiers could be changing the data.

For now interface locking happens for render job only in case
"Lock Interface" checkbox is enabled.

Currently this option would only make rendering thread-safe, but
in the future more benefits are possible to gain from it. Namely,
if we'll make renderer using it's own graph, this option would
allow to free memory used by 3D viewport graph, which would help
keeping memory usage low (or even would allow renderer not to
copy anything in this case).

Initially thought this change will also allow to free DMs used
by viewport, but we couldn't actually do this. This is because
of modifiers which uses other objects (like boolean), They're
in fact using viewport DM. This is bad because of few reasons.

We currently need to have viewport DM when rendering.
And for sure even in background render viewport DMs are being
calculated. This sounds like 2x computing is needed: one is for
viewport DM and one is for RenderDM.

If we'll have local graphs, we'll be able to compute RenderDMs
only and store them in graph. This would require a bit more of
the memory, but would solve current issues with viewport DM
used for modifiers operands while rendering and it should give
quite noticeable speedup.

Other tools like backing would also benefit of this option,
but rather get approval of current way of locking first.

source/blender/windowmanager/WM_api.h

@@ -412,6 +412,9 @@ void        WM_main_playanim(int argc, const char **argv);
 /* debugging only, convenience function to write on crash */
 int write_crash_blend(void);
 
+			/* Lock the interface for any communication */
+void        WM_set_locked_interface(struct wmWindowManager *wm, bool lock);
+
 #ifdef __cplusplus
 }
 #endif