(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.

libobs/obs.h

@@ -380,8 +380,11 @@ EXPORT bool obs_enum_encoder_types(size_t idx, const char **id);
 /** Enumerates all available service types. */
 EXPORT bool obs_enum_service_types(size_t idx, const char **id);
 
-/** Gets the main graphics context for this OBS context */
-EXPORT graphics_t obs_graphics(void);
+/** Helper function for entering the OBS graphics context */
+EXPORT void obs_enter_graphics(void);
+
+/** Helper function for leaving the OBS graphics context */
+EXPORT void obs_leave_graphics(void);
 
 /** Gets the main audio output handler for this OBS context */
 EXPORT audio_t obs_audio(void);