backend: add shadow_from_mask based implementation of render_shadow

If the backend implements shadow_from_mask then it doesn't need to
implement render_shadow.

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

src/backend/backend.h

@@ -215,12 +215,18 @@ struct backend_operations {
 	void *(*bind_pixmap)(backend_t *backend_data, xcb_pixmap_t pixmap,
 	                     struct xvisual_info fmt, bool owned);
 
+	/// Create a shadow context for rendering shadows with radius `radius`.
+	/// Default implementation: default_backend_create_shadow_context
 	struct backend_shadow_context *(*create_shadow_context)(backend_t *backend_data,
 	                                                        double radius);
+	/// Destroy a shadow context
+	/// Default implementation: default_backend_destroy_shadow_context
 	void (*destroy_shadow_context)(backend_t *backend_data,
 	                               struct backend_shadow_context *ctx);
 
-	/// Create a shadow image based on the parameters
+	/// Create a shadow image based on the parameters. Resulting image should have a
+	/// size of `width + radisu * 2` x `height + radius * 2`. Radius is set when the
+	/// shadow context is created.
 	/// Default implementation: default_backend_render_shadow
 	///
 	/// Required.
@@ -228,7 +234,8 @@ struct backend_operations {
 	                       struct backend_shadow_context *ctx, struct color color);
 
 	/// Create a shadow by blurring a mask. `size` is the size of the blur. The
-	/// backend can use whichever blur method is the fastest.
+	/// backend can use whichever blur method is the fastest. The shadow produced
+	/// shoule be consistent with `render_shadow`.
 	///
 	/// Optional.
 	void *(*shadow_from_mask)(backend_t *backend_data, void *mask,

src/backend/backend_common.c

@@ -311,6 +311,21 @@ void *default_backend_render_shadow(backend_t *backend_data, int width, int heig
 	return ret;
 }
 
+/// Implement render_shadow with shadow_from_mask
+void *
+backend_render_shadow_from_mask(backend_t *backend_data, int width, int height,
+                                struct backend_shadow_context *sctx, struct color color) {
+	region_t reg;
+	pixman_region32_init_rect(&reg, 0, 0, (unsigned int)width, (unsigned int)height);
+	void *mask = backend_data->ops->make_mask(
+	    backend_data, (geometry_t){.width = width, .height = height}, &reg);
+	pixman_region32_fini(&reg);
+
+	void *shadow = backend_data->ops->shadow_from_mask(backend_data, mask, sctx, color);
+	backend_data->ops->release_image(backend_data, mask);
+	return shadow;
+}
+
 struct backend_shadow_context *
 default_create_shadow_context(backend_t *backend_data attr_unused, double radius) {
 	auto ret =

src/backend/backend_common.h

@@ -64,6 +64,11 @@ bool default_is_frame_transparent(void *, win *, void *);
 
 void *default_backend_render_shadow(backend_t *backend_data, int width, int height,
                                     struct backend_shadow_context *sctx, struct color color);
+
+/// Implement `render_shadow` with `shadow_from_mask`.
+void *
+backend_render_shadow_from_mask(backend_t *backend_data, int width, int height,
+                                struct backend_shadow_context *sctx, struct color color);
 struct backend_shadow_context *
 default_create_shadow_context(backend_t *backend_data, double radius);