* glium -> 0.23

This reduces the compile time, ensures that depending crates don't get
features forced onto them that they don't need / want / can't have and
works around a cargo bug around features.

Provided by rustc-hash crate ~30% gpu-cache speed boost
Minor refactor to rasterizer code

Small key hash speed improves draw benchmarks by 9-65%

Mirror the reference tests in function with fast byte-insert callbacks
verified by blake2 hash.

Replace lossless storage of scale/subpixel offset +
BTreeMapsearchformatching glyph -> lossy storage of
scale/offsetproportionaltotolerances + a HashMap lookup.

Allows phase 1 of `cache_queued` work to be spread over available cores.

Add `approx` crate to deal with float equality

Commit 2989ad86 exposed the `units_per_em` function from the stb_truetype
crate, but did not update the Cargo.toml to reflect that it was added
in the 0.2.2 patch of stb_truetype.

Projects already using a "0.2" semver in their Cargo.toml would not
bother to update from 0.2.0 or 0.2.1, and results in a compile error.