shape: More tweaks to space handling
It is a difficult dance between HarfBuzz and pango which glyphs we let hb synthesize, and which ones we need to fake ourselves.
HarfBuzz knows how to create other spaces from 0x20, so if the font is lacking spaces, we want to tell HarfBuzz that we don't have that glyph except for 0x20 where we need to fake one, and provide HarfBuzz with a suitable width.