gdk: Add a scale property to textured cursors

On outputs with a scale > 1, named cursors can use a matching scale image from the matching theme, but currently there is no way to achieve a good-looking textured cursor on such outputs.

This commit adds a new scale argument to gdk_cursor_new_from_texture (), describing the GdkTexture it is created from. The fallback mechanism is then used to decide which scale to use, so for instance a cursor of scale=2 will only be accepted on an output of scale≥2, while a cursor of scale=1 will get used on any output.