(API Change) Remove obs_graphics()

API Removed:
- graphics_t obs_graphics();
Replaced With:
- void obs_enter_graphics();
- void obs_leave_graphics();

Description:
  obs_graphics() was somewhat of a pointless function.  The only time
that it was ever necessary was to pass it as a parameter to
gs_entercontext() followed by a subsequent gs_leavecontext() call after
that.  So, I felt that it made a bit more sense just to implement
obs_enter_graphics() and obs_leave_graphics() functions to do the exact
same thing without having to repeat that code.  There's really no need
to ever "hold" the graphics pointer, though I suppose that could change
in the future so having a similar function come back isn't out of the
question.

Still, this at least reduces the amount of unnecessary repeated code for
the time being.

plugins/win-capture/window-capture.c

@@ -334,9 +334,9 @@ static void wc_destroy(void *data)
 		bfree(wc->class);
 		bfree(wc->executable);
 
-		gs_entercontext(obs_graphics());
+		obs_enter_graphics();
 		effect_destroy(wc->opaque_effect);
-		gs_leavecontext();
+		obs_leave_graphics();
 
 		bfree(wc);
 	}
@@ -413,7 +413,7 @@ static void wc_tick(void *data, float seconds)
 		return;
 	}
 
-	gs_entercontext(obs_graphics());
+	obs_enter_graphics();
 
 	GetClientRect(wc->window, &rect);
 
@@ -438,7 +438,7 @@ static void wc_tick(void *data, float seconds)
 	}
 
 	dc_capture_capture(&wc->capture, wc->window);
-	gs_leavecontext();
+	obs_leave_graphics();
 }
 
 static void wc_render(void *data, effect_t effect)