Pad gpu cache glyphs II

Increase texture size a little to account for padding

Introduce `CacheBuilder`

Pack glyphs with a single extra zero-alpha pixel on the bottom & right.

Inline point(x, y) & vector(x, y)

gpu_cache: fix tree removal on row clear

Add Font::from_bytes

Add `rusttype::Error`, for `FontCollection` and `Font` operations.

More gpu_cache improvements

Add `approx` crate to deal with float equality

Replace CodepointOrGlyphId enum with IntoGlyphId trait.

Remove gh-pages docs, add nightly ci testing

Use fnv hashing for gpu_cache small keys maps

Feature/issue 86 expose font names