Metrics global to the font, i.e. not specific to single
glyphs. The font height is defined as
ascent+descent+internalLeading, and therefore not explicitely
included here.
Please note that when querying FontMetrics from an XCanvasFont
interface, all values here are given relative to the font cell
size. That means, the referenceCharWidth and/or
ascent+descent+internalLeading will approximately (rounded to
integer device resolution, or exactly, if fractional font
rendering is enabled) match the referenceAdvancement/cellSize
members of the FontRequest for which the XCanvasFont was
queried. Please be aware that the values returned in this
structure only map one-to-one to device pixel, if the combined
rendering transformation for text output equals the identity
transformation. Otherwise, the text output (and thus the resulting
metrics) will be subject to that transformation. Depending on the
underlying font technology, actual device output might be off by
up to one device pixel from the transformed metrics.
This value specifies the reference character width of the
font.
It's roughly equivalent to the average width of all
characters, and if one needs a font with double character
width, the referenceCharSize should be doubled.
