CJK vertical/horizontal detection angle changes based on aspect ratio

[ChrisLoer]

Porting is great for finding bugs!

The logic here tries to determine if the label has crossed a 45 degree angle by seeing if the rise is greater than the run:

https://github.com/mapbox/mapbox-gl-js/blob/master/src/symbol/projection.js#L266

The problem is that x and y are in GL coordinates, not pixel coordinates, so they're not directly comparable unless the viewport is exactly square. At a 2:1 aspect ratio, the angle at which the glyphs flip will be cut down to 22.5 degrees, which doesn't look totally broken but does look a little odd.

Unfortunately for us, our render tests for Chinese are all done on perfectly square canvases, so we didn't notice. Needs a regression test.

/cc @ansis @1ec5