libobs: Filter using premultiplied alpha

Convert incoming straight alpha color to premultiplied at filter begin,
and process premultiplied value at filter end.

If direct rendering is allowed, we assume the input source outputs
premultiiplied alpha. If not, that source will need to be updated.

libobs/obs-source.c

@@ -3701,10 +3701,11 @@ bool obs_source_process_filter_begin(obs_source_t *filter,
 		filter->filter_texrender =
 			gs_texrender_create(format, GS_ZS_NONE);
 
-	gs_blend_state_push();
-	gs_blend_function(GS_BLEND_ONE, GS_BLEND_ZERO);
-
 	if (gs_texrender_begin(filter->filter_texrender, cx, cy)) {
+		gs_blend_state_push();
+		gs_blend_function_separate(GS_BLEND_SRCALPHA, GS_BLEND_ZERO,
+					   GS_BLEND_ONE, GS_BLEND_ZERO);
+
 		bool custom_draw = (parent_flags & OBS_SOURCE_CUSTOM_DRAW) != 0;
 		bool async = (parent_flags & OBS_SOURCE_ASYNC) != 0;
 		struct vec4 clear_color;
@@ -3718,10 +3719,10 @@ bool obs_source_process_filter_begin(obs_source_t *filter,
 		else
 			obs_source_video_render(target);
 
+		gs_blend_state_pop();
+
 		gs_texrender_end(filter->filter_texrender);
 	}
-
-	gs_blend_state_pop();
 	return true;
 }
 
@@ -3748,6 +3749,9 @@ static void obs_source_process_filter_tech_end_internal(
 
 	const char *tech = tech_name ? tech_name : "Draw";
 
+	gs_blend_state_push();
+	gs_blend_function(GS_BLEND_ONE, GS_BLEND_INVSRCALPHA);
+
 	if (can_bypass(target, parent, parent_flags, filter->allow_direct)) {
 		render_filter_bypass(target, effect, tech);
 	} else {
@@ -3757,6 +3761,8 @@ static void obs_source_process_filter_tech_end_internal(
 		}
 	}
 
+	gs_blend_state_pop();
+
 	gs_set_linear_srgb(previous);
 }