Don't change fonts just for space characters

Back in the days we used to always use the font from surrounding runs
for any space characters.  I changed that when I found that rendering
MathML results in hexboxes when a font doesn't have the space characters
needed...

Forward to these days: HarfBuzz will silently compatibility-decompose
any space character to the ASCII space.  And looks like it's important
for Mongolian to not break runs around NNBSP.  As such, go back to never
choosing a font just for space characters.

Seems to work, though more testing is needed.

Bug 701652 - wrong shaped classic Mongolian word suffixes