This is a cleanup without any real savings (yet). Previously, the
implementation would allocate a new glyph, then (often) find it in
the cache, and immediately discard the allocated object. This
re-organization first uses a new FindGlyphByHash function and only
allocates the glyph if nothing is found.
This isn't a real savings yet, since FindGlyphByHash currently still
does a temporary glyph allocation, but this is expected to be replaced
immediately as we switch to an alternate hashing mechanism (SHA1).
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