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
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.