backend: add custom shader image property

Signed-off-by: Yuxuan Shui <yshuiv7@gmail.com>

src/backend/backend.c

@@ -383,6 +383,10 @@ void paint_all_new(session_t *ps, struct managed_win *t, bool ignore_damage) {
 				    ps->backend_data, IMAGE_PROPERTY_BORDER_WIDTH,
 				    w->win_image, &border_width);
 			}
+
+			ps->backend_data->ops->set_image_property(
+			    ps->backend_data, IMAGE_PROPERTY_CUSTOM_SHADER, w->win_image,
+			    w->fg_shader ? (void *)w->fg_shader->backend_shader : NULL);
 		}
 
 		if (w->opacity * MAX_ALPHA < 1) {

src/backend/backend.h

@@ -70,6 +70,9 @@ enum image_properties {
 	// Border width
 	// 1 int, default: 0
 	IMAGE_PROPERTY_BORDER_WIDTH,
+	// Custom shader for this window.
+	// 1 pointer to shader struct, default: NULL
+	IMAGE_PROPERTY_CUSTOM_SHADER,
 };
 
 enum image_operations {

src/backend/backend_common.c

@@ -452,6 +452,7 @@ bool default_set_image_property(backend_t *base attr_unused, enum image_properti
 	case IMAGE_PROPERTY_CORNER_RADIUS: tex->corner_radius = dargs[0]; break;
 	case IMAGE_PROPERTY_MAX_BRIGHTNESS: tex->max_brightness = dargs[0]; break;
 	case IMAGE_PROPERTY_BORDER_WIDTH: tex->border_width = *(int *)arg; break;
+	case IMAGE_PROPERTY_CUSTOM_SHADER: break;
 	}
 
 	return true;