Deduplicate and clarify the code a bit

Makes it clear that the engine for font with no Latin WS support
won't be cached as "shared" engine and thus shouldn't be normally
expected in the cache, so don't even try to find it there.

Change-Id: I9e6275b4919607f4057a193b446825c98932bd23
Reviewed-by: Eskil Abrahamsen Blomfeldt <eskil.abrahamsen-blomfeldt@theqtcompany.com>