Fix int16_t for glyphs, const glyphs, and clarify glyph loop.
Several places in the PDF code are using int16_t for glyphs. With
newer NotoSans fonts, all possible glyph ids are being used, so this
can lead to problems.
The PDF glyphs from text code returns the text for the glyphs if the
encoding is for glyphs. However, it returns this using an unsafe const
cast which is hiding possible bugs and preventing correct use of const
in other places.
The way the glyph loop in SkPDFDevice::drawPosText is written uses a
'--i' in the loop, which makes it appear this can loop forever. I don't
believe it can, but it is an unecessary code folding. We should also at
least assert the forward progress correctness here.