This value corresponds to the font height in Western scripts,
but is independent of the writing direction (see
FontRequest::IsVertical below). That
means, the value specified here is always measured orthogonal
to the text advancement (height for horizontal writing, and
width for vertical writing).
When this value is negative, its absolute value is taken as
the character size of the font. If this value is positive,
it's taken as the cell size of the font.
This member and the referenceAdvancement member are mutually
exclusive, one of them has to be set to 0 (which means don't
care).
For distorted fonts, the render transformation must be
used. That is, the size specified here corresponds to device
pixel only if the combined render transformation during text
output equals the identity transform. This also applies to all
query methods, for both XCanvasFont and
XTextLayout.