If two marks want to ligate and they belong to different components of the
same ligature glyph, and said ligature glyph is to be ignored according to
mark-filtering rules, then allow.

Example Burmese senquence:

  U+1004,U+103A,U+1039,U+101B,U+103D,U+102D

Test font provided by Norbert Lindenberg.

Fixes https://github.com/behdad/harfbuzz/issues/545

This test font had a upem of 769, which results in rounding-related errors with
the FreeType font funcs.  Change the upem to 1024 to fix that.

Fixes https://github.com/behdad/harfbuzz/issues/201

We use three bits for lig_id these days, so we finally got a report of
two separate ligatures with the same lig_id happening adjacent to each
other, and then the component-handling code was breaking things.
Protect against that by ignoring same-lig-id but lig-comp=0 glyphs after
a new ligature.

Fixes https://github.com/behdad/harfbuzz/issues/198