Add get_outline_commands_uncached

[taj-p]

Intent

Add API to get the outline commands for a glyph without caching results into outline_command_cache.

Context

I am trying to build a font renderer for low powered and memory constrained devices. Since I am caching the results myself (storing the commands on the GPU for on-GPU rendering - e.g. the Dobbie Method), an uncached version of getting the outline commands allows me to save some memory and avoid a hashmap lookup.

Extra

I've amended the outline_command_cache value type and swash_outline_commands return type to Box<[T]> to:

• Signal that commands are immutable once created
• Ensure the Vec isn't storing any unused capacity (it isn't from my inspection, but, I think it helps enforce that requirement).
• Saves a tiny amount of memory since we no longer need to store the capacity value