Introduce optional smarter font merging with ContextFontMergingHEADdev

This introduces an optional, slightly more expensive approach
to font merging which takes the full string into account,
instead of just going character by character.

This addresses the issue that you may sometimes get multiple fonts
to cover one string of text in a single language. With Chinese,
this is especially an issue because many fonts will only support
parts of the very large character set.

The new algorithm detects if the string was incompletely covered
by the font and tries the fallback fonts in order to find the best
match. This is obviously more expensive, especially if no perfect
match is found and we have to check all the fallbacks in the list,
but it is opt-in and only enabled if the ContextFontMerging flag
is set.

Task-number: QTBUG-121131
Change-Id: I8c7874d0918640bd83418e3c4726c89f43a220a3
Reviewed-by: Eirik Aavitsland <eirik.aavitsland@qt.io>

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