Make `gpu_cache` optional

Add a simple new `image` example

Exposed units_per_em function in rusttype

Update rusttype version in README

This ensures the getting starting guide is accurate.

Add support for unscaled fonts

This enables a user to get the raw, unscaled font metrics
(which is needed for embedding fonts in PDFs, for example).

Also made it possible to multiply the VMetrics by an f32
in order to scale them.

This is useful if you have to access the actual vertices
of the glyph or calculate the UNSCALDED metrics of a glyph.

Partition gpu_cache glyph lookup by FontId-GlyphId

Improves gpu cache performance

Implement `Display` and `Error` traits for error types

Fix panic sorting PartialOrd

Fix `shape_with_offset` method to correctly return multiple contours

Fix font.glyph lifetimes allow proxy glyphs in cache

Avoid unnecessary boxing

Remove forked rust std libs

Dependency on these died with dd9cdc5d

Fix to glyph lifetime generation to be the font's lifetime, rather than
the reference to the font.
Remove forced .standalone() calls in cache to, allowing improved
performance for proxy glyphs.

Fix gpu_cache PGlyphSpec Ord,Eq implementations

Fix glyph ordering panic in gpu cache code caused by partial_cmp unwrap

Fix tests/benchmarks