[OS X] canvas for eye tracking calibration is incorrectly sized under certain circumstances

[knausj85]

I typically use my MacBook Pro with an external monitor with the laptop closed. My eye tracker is attached to the external monitor.

If I start talon with both monitors and then close the laptop monitor, the canvas for eye tracking calibration is incorrectly sized and it is not possible to calibrate.

MacBookPro16,1
LG UltraFine:
Resolution: 3840 x 2160 (2160p/4K UHD 1 - Ultra High Definition)

[knausj85]

FYI this also causes issues with e.g. window snapping positions

[lunixbochs]

just fixed as part of #382