[Interest] QFontEngineFT glyph problem

[Grey DeMonstr]

Hello. My application has the following issue: it reads the external font
files tens thousands times a minute. I'm using Qt 5.6.1, so I've checked
bugtracker and backported the https://bugreports.qt.io/browse/QTBUG-49452 fix
from Qt 5.6.2 branch. That helped a lot, but really not fully. Now font
files are read about a one thousand times per minute.
Looks like glyphs are going to cache but still re-read every time.

QFontEngineFT::Glyph *QFontEngineFT::loadGlyphFor(...)
{
...
    Glyph *glyph = glyphSet != 0 ? glyphSet->getGlyph(g, subPixelPosition)
: 0;
    if (!glyph || glyph->format != format || (!fetchBoundingBox &&
!glyph->data)) {
       ...
        glyph = loadGlyph(glyphSet, g, subPixelPosition, format, false);
      ...
    }
    return glyph;
}
Reason is: glyph->data is null at all times, so QTBUG-49452 fix checks it
and returns NULLPTR, but on the next pass we are loading this glyph again
and again.

What check or QFontCache improvement will be the best in this case?
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