I can't figure out whether the LSBearing bit (bit 1) of head.flags controls the horizontal metrics, or is simply an "efficiency" bit that represents all the LSB values in the 'htmx' table. Does:
(A) head.flags.LSBearing mean "ignore the per-glyph LSB values in the 'hmtx' table"? ... or
(B) Is it more of an advisory bit that certifies that the 'hmtx' LSB values all match the xMin values declared in the 'glyf' table for the corresponding glyph? ... or
(C) Some other, undreampt-of, interpretation.