[otfautohint] Consider adding flag to selectively skip overlap processing for some glyphs

[skef]

Related to #1728, some variable glyphs may be hinted better when overlap processing is skipped. (This will most likely be when overlap is in the area of independent "strokes"/graphical elements, such as the glyph in that issue.) We could add a flag to select specific glyphs to skip that processing for (or add the processing if it is turned off by default).