Don't change fonts just for space characters

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