Add static type for different coordinate systems and length units

[mbrubeck]

We currently use primitive types like int and f32 for measurements which may be in various coordinate planes with different origins and transforms (screen coordinates, window coordinates, framebuffer coordinates, page coordinates, layer coordinates) and in different units (CSS px, hardware pixels, device-independent "pixels", appunits).

We should add types for some or all of these, and statically-checked conversions between units and between coordinate systems. (Currently we have one such type, Au (appunit), which also defines conversions to untyped "px" and "pt" units.)

For comparison, here are some of the relevant types and functions in Gecko:

• http://dxr.mozilla.org/mozilla-central/source/layout/base/Units.h
• https://wiki.mozilla.org/Platform/GFX/APZ#Coordinate_systems