[Linux] fixed font rendering crash for certain fonts

face->size->metrics->ascender seems to be unreliable for some fonts,
additionally the freetype documentation says that it may be used
differently for different fonts.
Therefore it may happen that the ascender of a font face is less then
the glyphs bounding box resulting in accessing invalid memory.

The fix is to use the bbox attribute instead of the ascender
Merge pull request #2271 from Weeds/fontdimension-fix

fixed #1932: [Linux] fixed font rendering crash for certain fonts.
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