spice-widget: input and display enhancements on HiDPI displays
Allow guests to take full advantage of the extra pixel real-estate on a HiDPI display.
On HiDPI displays:
- Guest resizes to the actual pixel size on both Cairo and EGL display.
- Mouse cursor scales with guest display size (in both client and server mode). In other words, it should always be the same size as it is in the virtual display.
- Fixed incorrect mouse input coordinate (fixes #127 (closed))
This is a continuation of !56 (closed) and supersedes it. Tested on Plasma (Xorg) and Sway (Wayland).