backend: add new property: BORDER_COLOR

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

src/backend/backend.c

@@ -342,6 +342,15 @@ void paint_all_new(session_t *ps, struct managed_win *t, bool ignore_damage) {
 			ps->backend_data->ops->set_image_property(
 			    ps->backend_data, IMAGE_PROPERTY_CORNER_RADIUS, w->win_image,
 			    (double[]){w->corner_radius});
+			if (w->corner_radius) {
+				struct color border_color;
+				ps->backend_data->ops->read_pixel(
+				    ps->backend_data, w->win_image, 0, w->heightb,
+				    &border_color);
+				ps->backend_data->ops->set_image_property(
+				    ps->backend_data, IMAGE_PROPERTY_BORDER_COLOR,
+				    w->win_image, &border_color);
+			}
 		}
 
 		if (w->opacity * MAX_ALPHA < 1) {

src/backend/backend.h

@@ -58,6 +58,9 @@ enum image_properties {
 	// Gives the image a rounded corner.
 	// 1 double, default: 0
 	IMAGE_PROPERTY_CORNER_RADIUS,
+	// Border color
+	// 1 struct color, default: black
+	IMAGE_PROPERTY_BORDER_COLOR,
 };
 
 enum image_operations {

src/backend/backend_common.c

@@ -451,6 +451,7 @@ bool default_set_image_property(backend_t *base attr_unused, enum image_properti
 		break;
 	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_COLOR: tex->border_color = *(struct color *)arg; break;
 	}
 
 	return true;

src/backend/backend_common.h

@@ -41,6 +41,7 @@ struct backend_image {
 	// Effective size of the image
 	int ewidth, eheight;
 	bool color_inverted;
+	struct color border_color;
 };
 
 bool build_shadow(xcb_connection_t *, xcb_drawable_t, double opacity, int width,