When inline layout emits a whitespace chunk, it previously selected the
surrounding text's font without checking whether that font actually
contains a glyph for the whitespace codepoint. On pages that use
`@font-face` rules sharded by `unicode-range` (e.g. a Roboto webfont
split across one file for Cyrillic letters and another for basic Latin),
the shard covering the letters is picked for an adjacent space even
though the space codepoint lives in a different shard. HarfBuzz then
shapes the space with a font that has no glyph for it and emits
`.notdef`, rendering spaces as tofu boxes.
Check `contains_glyph(space_code_point)` on each candidate in
`font_for_space()` and fall through to
`FontCascadeList::font_for_code_point()` for the whitespace codepoint
when no surrounding font has the glyph.
Fixes whitespace rendering on web.telegram.org/a.
