gsk: Improve glyph serialization

gsk: Improve glyph serialization

The special case for ASCII glyphs is unfortunately not
working very well, because of an oversight in pango:
When I added subpixel positioning, I made pango_shape()
default to not rounding, and make PangoLayout call
pango_shape_with_flags() and pass the rounding information
down. The upshot is that we need to use the _with_flags
variant here and tell it to round position, so it matches
what the text node contains.