Add multi-monitor support

[James-Livesey]

To mirror a display (for this example, eDP-1 is primary and DP-3 is mirror):

xrandr --output eDP-1 --mode 2256x1504 --pos 0x0 --output DP-3 --mode 1920x1080 --pos 0x0 --transform 1.39,0,-209,0,1.39,0,0,0,1

Coordinate transformation matrix:

$$ \begin{pmatrix} S_w & 0 & P_x\\ 0 & S_y & P_y\\ 0 & 0 & 1\\ \end{pmatrix} $$

$S = S_w = S_h$ for letterboxing. For vertical letterbox bars then $S = \frac{1,504}{1,080} = 1.39$.

$P_x$ and $P_y$ should be negative.