linux-pipewire: Use premultiplied alpha

Pipewire desktop portal capture assumes premultiplied alpha, so make
sure to use the right blend equation to make translucent pixels work
correctly.

This is still broken for emissive pixels (alpha < color) since OBS seems
to unpremultiply at some point during blending, but it works properly
for translucent pixels (emissive pixel support requires an end-to-end
premultiplied pipeline).

plugins/linux-pipewire/pipewire.c

@@ -1090,6 +1090,18 @@ void obs_pipewire_video_render(obs_pipewire *obs_pw, gs_effect_t *effect)
 	rotated = push_rotation(obs_pw);
 
 	flip = get_buffer_flip(obs_pw);
+
+	/* There is a SPA_VIDEO_FLAG_PREMULTIPLIED_ALPHA flag, but it does not
+	 * seem to be fully implemented nor ever set. Just assume premultiplied
+	 * always, which seems to be the default convention.
+	 *
+	 * See https://gitlab.freedesktop.org/pipewire/pipewire/-/issues/3126
+	 *
+	 * The cursor bitmap alpha mode is also not specified, and the
+	 * convention there also seems to be premultiplied. */
+	gs_blend_state_push();
+	gs_blend_function(GS_BLEND_ONE, GS_BLEND_INVSRCALPHA);
+
 	if (has_effective_crop(obs_pw)) {
 		gs_draw_sprite_subregion(obs_pw->texture, flip, obs_pw->crop.x,
 					 obs_pw->crop.y, obs_pw->crop.width,
@@ -1115,6 +1127,8 @@ void obs_pipewire_video_render(obs_pipewire *obs_pw, gs_effect_t *effect)
 
 		gs_matrix_pop();
 	}
+
+	gs_blend_state_pop();
 }
 
 void obs_pipewire_set_cursor_visible(obs_pipewire *obs_pw, bool cursor_visible)