We always create a SSD for 0x0 window since decorations are usually
requested before a window is mapped. Thus the sizes of some buffers/rects
like edge shadows could be negative, which is asserted in wlroots 0.19.
For example, variables:
`theme->window_inactive_border_color`
`theme->window_active_border_color`
are converted to:
`theme->window[THEME_INACTIVE].border_color`
`theme->window[THEME_ACTIVE].border_color`
This fixes the ugly look of SSD for tiny windows (e.g. "xterm -geometry
1x1") due to the early return in `ssd_update_geometry()`. Now SSDs are
rendered correctly for those windows by hiding some buttons when the
window width is smaller than the total width of buttons. Additionally for
windows smaller than (button width)*2, the corners are un-rounded so a
small titlebar can be rendered with a scene-rect.
