-x, -y screen positioning

[youbitchoc]

Dzen allows negative -x and -y values to position from the opposite side, so our -b can be replaced with -y -1.
However, the bar seems to be prevented from starting beyond the bottom border.
So y -$H will, for $H < $HEIGHT, keep the bar right at the bottom, and otherwise leave a $((H - HEIGHT)) gap.

I find this behaviour odd. What we could instead do is reflect the height (i.e. implicitly raise by $HEIGHT if anchored to bottom) such that dtao -y $H and dtao -y -$H will both be $Hpx from their respective edge regardless of $HEIGHT.

Of course, this is incompatible with dzen. There's also the question of whether flipping the y should start from '-1' or '-0', but that's just a minor implementation detail.

will probably implement the offsetting with zwlr_layer_surface_v1 set_margin

Opinions?