gpu_cache: return CachedBy info

Returns CachedBy enum for successful cache_queued calls.

This enum distinguishes between whether we were able to fit in any new glyphs no destructively (CachedBy::Adding) or whether the cache had to be re-packed to fit in the queue (CachedBy::Reordering). This is necessary if users want to hold onto texture positions from previous cache_queued calls, this allows optimising by avoiding rect_for calls.