fix(gridScale): use whole numbers for `gridPix`

[Iron-E]

Foundry doesn't like using decimals for the grid scale, so we have to round them before using them.

This fix was initially proposed by @u2mad.

Closes #20.

[Iron-E]

@UberV have you had a chance to review?

[jbhaywood]

I've taken over this module and applied this fix to the newest release. https://github.com/jbhaywood/scaleGrid/releases/tag/v1.1.0

[Iron-E]

Thank you for taking over on the project!