As crengine/FreeType can't distinguish sans serif
from serif, this is only useful to detect if a font
is monospace or not. Might be useful for frontends.

Should trigger a rerendering when we change the
bias of some fonts.
Also fix GetFontListHash() which was always
returning 0 since ages...

This should allow another monospace font, set with a bias,
to be used for <pre> and <code> (as with anything having
"font-family: monospace").

So that when using a main font with a small x-height,
one can avoid having tall inline <code> bits, and
adjust them (manually) to fit the size of the main font
or to be a bit smaller to help distinguishing them.

Having all fallback font size having the same x-height
should make lowercase letters picked from them less
noticable (this can be noticable with greek words,
as latin fonts often have only a subset of greek
letters, related to maths).

EPUBs can have both, and it seems that doing it
per XHTML specs is the right thing to do.