Grid and background aren't aligned

[dacap]

Reported here: https://community.aseprite.org/t/zoom-grid-vs-checkered-background/16566

Create a new sprite with a checkered background and grid of 64x64, and then zoom out.

[Proggle]

at 33.3% the grid also diverges significantly from the underlying image - try setting a 128x128 grid on a 1024x1024 image.

If you're trying to lay down tiles or whatever it becomes a huge pain for the grid to diverge this far from the real location

[dacap]

Fixed with ff14066

@martincapello remember to include the (fix #issueNumber) in the commit description