`view->ssd_enabled && view->ssd == NULL` is possible during the client is
in fullscreen. So we need to check if `view->ssd` is NULL first in
`ssd_update_window_icon()`.
The default `titleLayout` is updated to `icon:iconify,max,close` which
replaces the window menu button with the window icon.
When the icon file is not found or could not be loaded, the window menu
icon as before is shown.
The icon theme can be selected with `<theme><icon>`.
This commit adds libsfdo as an optional dependency. `-Dicon=disabled` can
be passsed to `meson setup` command in order to disable window icon, in
which case the window icon is always replaced with a window menu button.
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}.
