Typeface::try_load_from_externally_owned_memory() relies on that
external owner keeping the memory around. However, neither WOFF nor
WOFF2 do so - they both create separate ByteBuffers to hold the TTF
data. So, rename them to make it clearer that they don't have any
requirements on the byte owner.
This change removes wrappers inherited from Gfx::Typeface for WOFF and
WOFF2 fonts. The only purpose they served is owning of ttf ByteBuffer
produced by decoding a WOFF/WOFF2 font. Now new FontData class is
responsible for holding ByteBuffer when a font is constructed from
non-externally owned memory.
