In Qt 6, we deferred getting the ascent/descent of the font
until requested. This was used to calculate the cache cost
of the font engine, so as an unintended result, the DirectWrite
engines were seen as extremely cheap in the font cache, and
we would accumulate thousands of them before triggering a flush.
The cache_cost is just a heuristic and does not need to be accurate.
For most engines, we should just flush when the number of cached fonts
exceed 256, so we use the basic em square of a lowercase letter as the
cost here instead.
Pick-to: 6.5 6.6
Change-Id: I73a65cab38adfd9ef56e436f311a7d1a2d7fbf41
Reviewed-by: Allan Sandfeld Jensen <allan.jensen@qt.io>
db6f7908d0