wayland: Support scaling of theme based cursors

wayland: Support scaling of theme based cursors

Support scaling of cursors created from themes. The default scale is
always 1, but if the pointer cursor surface enters an output with a
higher scale, load the larger version of the cursor theme and use the
image from that theme.

This assumes the theme size is set to one that fits with an output scale
= 1.

https://bugzilla.gnome.org/show_bug.cgi?id=746141