Fix variable anchors when DesignSpace source specifies a layer

[khaledhosny]

The anchor position was always taken from the default layer, ignoring the specified layerName.

[anthrotype]

good catch! this is basically the continuation of #840 but for the sparse UFO layers

@khaledhosny can you please also test that my assumption in googlefonts/fontmake#1090 (comment) is true?

[anthrotype]

can you please also test that my assumption in googlefonts/fontmake#1090 (comment) is true?

I just tried to remove the anchor "top" from the intermediate glyph "e" in the LayerFont.ufo's "Medium" layer used for the VF integration tests and indeed the intermediate glyph is skipped when producing the mark2base lookup.

  feature mark {
      lookup mark2base {
          pos base e
-             <anchor (wght=350:314 wght=450:403 wght=625:315) (wght=350:556 wght=450:567 wght=625:644)> mark @MC_top;
+             <anchor (wght=350:314 wght=625:315) (wght=350:556 wght=625:644)> mark @MC_top;
      } mark2base;

  } mark;