(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.

test/test-input/test-filter.c

@@ -15,12 +15,12 @@ static void filter_destroy(void *data)
 	struct test_filter *tf = data;
 
 	if (tf) {
-		gs_entercontext(obs_graphics());
+		obs_enter_graphics();
 
 		effect_destroy(tf->whatever);
 		bfree(tf);
 
-		gs_leavecontext();
+		obs_leave_graphics();
 	}
 }
 
@@ -29,7 +29,7 @@ static void *filter_create(obs_data_t settings, obs_source_t source)
 	struct test_filter *tf = bzalloc(sizeof(struct test_filter));
 	char *effect_file;
 
-	gs_entercontext(obs_graphics());
+	obs_enter_graphics();
 
 	effect_file = obs_module_file("test.effect");
 
@@ -41,7 +41,7 @@ static void *filter_create(obs_data_t settings, obs_source_t source)
 		return NULL;
 	}
 
-	gs_leavecontext();
+	obs_leave_graphics();
 
 	UNUSED_PARAMETER(settings);
 	return tf;