Transparency not always rendered

[VSharapov]

Reproduced on my Chrome Version 67.0.3396.62 (Official Build) (64-bit) on Fedora 28

Happens quite often (>10% of the time) when using the scrollwheel to zoom quickly.

Actually, just reproduced on my my Firefox 60.0.1 (64-bit) on Fedora 28 using the scrollwheel in the hex size field. Same thing - single steps work fine, but scrolling fast will neglect transparency.

[VSharapov]

It looks like the cause might be rendering over and over again - it's queuing. Fix might be as easy as clearing the canvas before drawing the scene, but the better fix would be to interrupt rendering.

[VSharapov]

Strange, cannot reproduce with just one monitor.