MarkFeatureWriter's markClasses inserted after manual mkmk features

[simoncozens]

Investigating notofonts/noto-source#377, I found that NotoLoopedThai.glyphs doesn't compile because it has a manually written mkmk. The automatic mkmk feature is correctly written to the feature file above this, but the mark classes don't appear until afterwards:

feature mkmk {     # THIS IS ufo2ft'S MKMK
    lookup mark2mark_bottomright {
        @MFS_mark2mark_bottomright = [saraU-thai saraU-thai.small saraUu-thai saraUu-thai.small phinthu-thai phinthu_saraU-thai phinthu_saraUu-thai];
        lookupflag UseMarkFilteringSet @MFS_mark2mark_bottomright;
        pos mark saraU-thai
            <anchor 196 -299> mark @MC_bottomright;
...
} mkmk;

feature mkmk {   # THIS IS THE FONT'S MKMK
    lookup combinewithdoublemacron {
        ### move doubleMacron up if preceded by Thai marks above
        pos @ThaiAboveMarks [doublemacroncomb doublemacroncomb.wide]' <0 190 0 0>;
...
} mkmk;

...

markClass macronbelowcomb <anchor 177 0> @MC_bottom;
markClass macronbelowcomb.alt <anchor 0 0> @MC_bottom;
...
feature mark {
    lookup mark2base {
        pos base koKai-thai
            <anchor 276 0> mark @MC_bottom
            <anchor 466 0> mark @MC_bottomright
...

Looks like ufo2ft is assuming that mark will always appear first.

[anthrotype]

closing as fixed by #505