SIMD additions

[taylorswift]

@scanon

As a side note, representing an image size as as SIMD vector seems at first glance like an extremely odd use case--what is the SIMD representation buying you over a tuple or array? Does it make sense to perform an elementwise sum of image sizes? Are height and width even the same thing? This seems like a recipe for subtle errors.

It’s useful for computing offsets/flattened positions, mainly through vector addition and subtraction. In the case of text shaping, having SIMD bitwise right shift is also useful with text shaping APIs, as a lot of them will give 2D coordinates and glyph image dimensions in 1/64s.