ssd: scale down button icons if necessary

ssd: scale down button icons if necessary

This allows using large XBM button icons (larger than
the button width / height) for high and mixed DPI usecases.

Instead of scaling up the low pixel icons when moving a window to a
high DPI output we scale the icons down when moving to a non-scaled
output. The user is still responsible to create / use larger XBM
icons for their theme.

Partly fixes #609