This patch adds support for observing multiple slices of the desktop background.
This is used for the tray so that it doesn't have to rely on the bar's rect to
get the desktop background. In particular, it now handles the case where the
tray is not contained fully within the bar's outer rect (for example, when using tray-offset-{x,y})
Co-Authored-By: bennofs <benno.fuenfstueck@gmail.com>
We need to use positions relative to the position of the bar for indexing into
the background image slice, but the code used absolute ones.
This worked fine as long as absolute positions are the same as relative
positions (this is the case for a bar located at (0,0), so if bottom = false).
But for bottom bars (where the bar position is not (0,0)) this was wrong which
caused the tray background to be black (out of bounds for the background slice).
