ssd: limit icon size to ~85% of window_button_width

ssd: limit icon size to ~85% of window_button_width

This ensures that the icon doesn't push right up to the window edge
(or left-aligned title) when using a large font or narrow button width.
In the default config (with 10pt font), it makes no difference since
the limiting factor on the icon size is the titlebar height anyway.

I spent way too much time over-thinking how to compute the padding.
I think 2px on each side is reasonable (and it should be equal on each
side), so window_button_width/10 (rounded down) should be fine.
Eventually the padding should probably be configurable anyway.