A cursor is an image, with an optional x,y hotspot.
We know that a CursorStyleValue's bitmap never needs to change size, so
we create the ShareableBitmap once and then cache it, so that we don't
have to repeatedly create an FD for it or do the work of painting.
To avoid repainting that bitmap, we cache the values that were used to
create it - what currentColor is and its length resolution context -
and only repaint when those change.