Call SalLayout::GetNextGlyphs with one glyph at a time while collecting
glyphs to draw, which allows us to simplify the code a bit and helps
simplify my next patch.

Change-Id: I5c9ddb7469301d8fbc6a15695c01af7b0879685a
Reviewed-on: https://gerrit.libreoffice.org/53384Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Khaled Hosny <khaledhosny@eglug.org>