[makeotfexe] Improve merging of SinglePos chain targets

[brawer]

Currently, makeotf combines the chain targets for the following feature into the SinglePos lookups {A:-40, B:-40, C:-40} and {A:-111, B:-40}. The duplicated entry for B seems redundant. Instead of putting B into both SinglePos chain targets with the same value, makeotf could emit {A:-40, B:-40, C:-40} and {A:-111} to save some space.

feature test {
    pos X [A-B]' -40 B' -40 A' -40 Y;
    pos X A' -111 Y;
    pos X B' -40 A' -111 [A-C]' -40 Y;
} test;

This is definitely a contrived test case, and I’m not sure how wasteful the current encoding is in practice. Just noticed this while working on fonttools/fonttools#514.