[Bari-ye] Collision rects and Multi-anchor attachment

[be5invis]

Very rough thoughts, basic idea:

• Glyphs can define various rects, and also corresponding "collision group" (an uint16_t).
• A multi-anchor attachment lookup will define a series of anchors, and when performing attachment, the shaper would find the first anchor that the rects sharing same collision group in the mark and the bases (or base-marks. The glyphs that should be considered as "bases" are selected using a mask, pretty like the flags in the lookup flags) won't collide.

DISCLAIMER: This is NOT a proposal from Microsoft, just from me.

[tiroj]

cf. Page 39 of my 2014 TypeCon presentation on Problems of Adjacency