libobs: Use default blend state with scenes

Instead of manually setting the blend state to the desired values, use
gs_reset_blend_state to ensure we have the default blend state (which
for color is a typical srcalpha/invsrcalpha alpha blending operation,
then the alpha channels are added together).

This fixes an issue where filtered scenes would look strange due to the
fact that alpha was not being blended properly.

libobs/obs-scene.c

@@ -308,7 +308,7 @@ static void scene_video_render(void *data, gs_effect_t *effect)
 	item = scene->first_item;
 
 	gs_blend_state_push();
-	gs_blend_function(GS_BLEND_SRCALPHA, GS_BLEND_INVSRCALPHA);
+	gs_reset_blend_state();
 
 	while (item) {
 		if (obs_source_removed(item->source)) {