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.
Before this commit, the backgrounds of titlebar corners were tagged as
LAB_SSD_PART_CORNER_TOP_{LEFT,RIGHT}, so the cursor shape on titlebar
corners without buttons were north-west or north-east.
This commit fixes it by tagging those backgrounds as
LAB_SSD_TITLEBAR_CORNER_{LEFT,RIGHT}.
