Count baseGlyphCount once for scripts without typefaces

Characters from complex scripts without dedicated typefaces were
getting baseGlyphCount counted twice: once in shapeFontRun() and
then a second time during SkScalerContext::getNextContext()
chaining.

This was a problem for text runs beginning with full-width Latn
characters - shapeFontRun() would get the base glyph count for
the typeface containing those characters, then realise it had no
typeface specified for the Latn script, resetting the typeface
to default without resetting the base glyph count along with it.
Mojibake full-width Latn characters, ho!

The same issue also affected Cherokee text, which this fix also
addresses.

Bug: 8127795
Bug: 8034351
Change-Id: I058d81cffbbf86426cc3c5aafcc91a76f4fe6b62