Add x, y parameters to getBoundingClientRect

[aekobear]

getBoundingClientRect returns a DOMRect which is supposed to have an x and y property. This change makes getBoundingClientRect behave more reliably for code which expects its properties to have defined numerical values.

fixes #3188

[domenic]

Although the general plan for these not-very-good mocks is still up in the air (see #2751), this seems like a reasonable improvement assuming CI passes, so I'll merge it pending further discussion on that point.

[domenic]

Looks like this requires a few test updates.

[aekobear]

@domenic sorry for the delay on this- I had trouble getting tests to run locally.

I updated the "build" tests so they should pass now, but I couldn't figure out why "build-with-canvas" was failing with a segmentation fault. Let me know if this is expected, or if there is further action that needs to be done on my part.