Mark feature writer doesn't mkmk correctly when both abvm and mark scripts in font

[simoncozens]

Okay, this is slightly complicated, but when the attached Glyphs File
MarkMarkTest.glyphs.gz (a subset of Noto Sans Tagalog) is compiled with fontmake, the resulting feature file looks like this:

markClass uni1712 <anchor -171 693> @MC_Anchor2;

feature abvm {
    lookup abvm_mark2base {
        pos base uni170A
            <anchor 488 693> mark @MC_Anchor2;
    } abvm_mark2base;

    lookup abvm_mark2mark_Anchor2 {
        @MFS_abvm_mark2mark_Anchor2 = [uni1712];
        lookupflag UseMarkFilteringSet @MFS_abvm_mark2mark_Anchor2;
        pos mark uni1712
            <anchor 27 693> mark @MC_Anchor2;
    } abvm_mark2mark_Anchor2;

} abvm;

feature mark {
    lookup mark2base {
        pos base uni25CC
            <anchor 300 590> mark @MC_Anchor2;
    } mark2base;

} mark;

table GDEF {
    GlyphClassDef [uni170A uni25CC], [], [uni1712], [];
} GDEF;

Notice that we have mark-to-base and mark-to-marks rule in the Tagalog part of the font that goes through the abvm feature, but we only have mark-to-base, and no mark-to-mark, for the dotted circle bit, leading to:

$ shape master_ttf/MarkMarkTest-Regular.ttf -u 170a,1712,1712,25cc,1712,1712

I think there should be a mkmk feature produced as well.