Fix delay first time a font is used

Since 066daf750fcffff8eeae4d5749607501b9aa9a2f, we would mistakenly populate the
fallback list for all ensureAt(index) calls, even when index == 0. This index
indicates the main font, is always valid and does not require knowledge of any
fallbacks.

On Windows (and other platforms where QPlatformDatabase::fallbacksForFamily() is used)
this would cause all fonts on the system to be loaded and should only be done when
we actually need one of the fallbacks.

With the GDI font database, in which font loading is slow because we have to manually
read font data, this is especially bad. If we can later move to using DirectWrite
for this, we should be able to improve on it, but in any case there is no need to
pay the cost of the fallbacks when the application is just using the fonts it has
selected (memory-wise this is also a bad idea). On my machine a simple text layout
went from 370 ms to 37 ms.

[ChangeLog][Text] Fixed an issue where on some platforms, there would be a delay the
first time any font was used, sometimes causing a visible delay in the UI.

Fixes: QTBUG-71737
Pick-to: 5.15
Change-Id: Ie8ce9b73f02a0e5cf39a2b280968b89f4caaf39e
Reviewed-by: Lars Knoll <lars.knoll@qt.io>
Reviewed-by: Konstantin Ritt <ritt.ks@gmail.com>

0 files changed, 0 insertions, 0 deletions