Offsets should be flipped vertically on macOS

[1ec5]

In the style specification, the y value of a property like fill-translate is defined as the number of pixels from the top. By contrast, offsets are normally measured from the bottom of the screen or view on macOS. In order to simplify calculations, the value exposed to the developer via the runtime styling API should be consistent with the system’s standard origin. This will become especially important once we replace the NSArray representation with a CGVector (#5947).

/cc @frederoni

[jfirebaugh]

Adding to the 3.4.0 milestone -- we need to implement this now or keep the existing behavior at least until a semver major version.

[1ec5]

This only applies to macOS; the origin is at the top-left on iOS. The macOS SDK hasn't reached v1.0.0 yet, so I'm not terribly concerned about backwards compatibility there.

[boundsj]

In that case, I'm going to remove the 3.4.0 milestone.

[1ec5]

Fixed in #7636 on the release-ios-v3.4.0 branch.