fix: re-draw already loaded center images on change

[mnahkies]

previously if the image was already loaded, changing the value of the QR code would make it disappear.

this change fixes that by immediately drawing the image if the complete property is true

may help with #64 - although as reported that was essentially the reverse of this situation :/

ref:

• https://developer.mozilla.org/en-US/docs/Web/API/HTMLImageElement/complete