Support bold/italic font rendering on macOS

This patch adds support for rendering italic fonts and bold fonts.

The `font` crate has a couple of new paradigms to support this: font
keys and glyph keys. `FontKey` is a lightweight (4 byte) identifier for
a font loaded out of the rasterizer. This replaces `FontDesc` for
rasterizing glyphs from a loaded font. `FontDesc` had the problem that
it contained two strings, and the glyph cache needs to store a copy of
the font key for every loaded glyph. `GlyphKey` is now passed to the
glyph rasterization method instead of a simple `char`. `GlyphKey`
contains information including font, size, and the character.

The rasterizer APIs do not define what happens when loading the same
font from a `FontDesc` more than once. It is assumed that the
application will track the resulting `FontKey` instead of asking the
font to be loaded multiple times.

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