(compose_glyph_string): Update prev->rface->rfont if the glyph prev is also supported by the current flt.
(compose_glyph_string): Update prev->rface->rfont if the
glyph prev is also supported by the current flt.

xmirror/m17n-lib,xmirror/m17n-lib,xmirror/m17n-lib,xmirror/m17n-lib