cursor: add wlr_cursor_unset_image()

It's pretty awkward to call wlr_cursor_set_image() with 6 zeroes.
Hide that awkwardness in wlroots.

include/wlr/types/wlr_cursor.h

@@ -147,6 +147,11 @@ void wlr_cursor_set_image(struct wlr_cursor *cur, const uint8_t *pixels,
 	int32_t stride, uint32_t width, uint32_t height, int32_t hotspot_x,
 	int32_t hotspot_y, float scale);
 
+/**
+ * Hide the cursor image.
+ */
+void wlr_cursor_unset_image(struct wlr_cursor *cur);
+
 /**
  * Set the cursor image from an XCursor theme.
  *

types/wlr_cursor.c

@@ -425,6 +425,10 @@ void wlr_cursor_set_image(struct wlr_cursor *cur, const uint8_t *pixels,
 	}
 }
 
+void wlr_cursor_unset_image(struct wlr_cursor *cur) {
+	wlr_cursor_set_image(cur, NULL, 0, 0, 0, 0, 0, 0);
+}
+
 void wlr_cursor_set_xcursor(struct wlr_cursor *cur,
 		struct wlr_xcursor_manager *manager, const char *name) {
 	struct wlr_cursor_output_cursor *output_cursor;