Move some default_* functions to backend_common.c

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

src/backend/backend.c

@@ -14,14 +14,6 @@ backend_info_t *backend_list[NUM_BKEND] = {
 #endif
 };
 
-bool default_is_win_transparent(void *backend_data, win *w, void *win_data) {
-	return w->mode != WMODE_SOLID;
-}
-
-bool default_is_frame_transparent(void *backend_data, win *w, void *win_data) {
-	return w->frame_opacity != 1;
-}
-
 region_t get_damage(session_t *ps) {
 	region_t region;
 	auto buffer_age_fn = backend_list[ps->o.backend]->buffer_age;

src/backend/backend_common.c

@@ -244,3 +244,11 @@ shadow_picture_err:
 
 	return false;
 }
+
+bool default_is_win_transparent(void *backend_data, win *w, void *win_data) {
+	return w->mode != WMODE_SOLID;
+}
+
+bool default_is_frame_transparent(void *backend_data, win *w, void *win_data) {
+	return w->frame_opacity != 1;
+}

src/backend/backend_common.h

@@ -19,3 +19,11 @@ solid_picture(session_t *ps, bool argb, double a, double r, double g, double b);
 
 xcb_image_t *
 make_shadow(xcb_connection_t *c, const conv *kernel, double opacity, int width, int height);
+
+/// The default implementation of `is_win_transparent`, it simply looks at win::mode. So
+/// this is not suitable for backends that alter the content of windows
+bool default_is_win_transparent(void *, win *, void *);
+
+/// The default implementation of `is_frame_transparent`, it uses win::frame_opacity. Same
+/// caveat as `default_is_win_transparent` applies.
+bool default_is_frame_transparent(void *, win *, void *);