spiv: Add zoom modes.

Finally spiv gets support for different zoom modes.

There are three zoom options:

* Fixed / Resizeable Window

* Turn On/Off Donwscale

* Turn On/Off Upscale

+ Maximal window size, which works best if you set it
  to your resolution - window decoration size.

What is still broken is fullscreen mode, we must not mess with window
size when we entered fullscreen (which needs a fix at a backend level
first.

Signed-off-by: Cyril Hrubis <metan@ucw.cz>