Optimise memory usage

[nicoburns]

Wuff currently buffers the entire compressed font in memory, again for the decompressed stream, and again for output buffer.

This is typically fine as web fonts aren't usually that big (~200-500kb is typical). However, some fonts are larger and for those it would be good to support a "streaming mode" that uses less memory.

• New overload OTSContext::Process to lower memory use processing WOFF2 khaledhosny/ots#299 may be useful inspiration.