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Add a reftest using an additional axis VM2B in the variabletest_box
font. The VM2B axis shifts the mark anchor to the left, leading to the
combining mark below box shifting right when the axis parameter is
adjusted from 0 to 1000. As the reference in the ref test, the font now
contains two additional glyphs N and O, which have fixed base anchors at
the middle and right position. A rendering of the M character showing
"m2b pos", followed by COMBINING SQUARE BELOW with VM2B 0, 500, 1000
applied must thus be equal to renderings of the N and O glyph with no
VM2B variation values applied.

BUG=693065
R=behdad, kojii

Review-Url: https://codereview.chromium.org/2850743002
Cr-Commit-Position: refs/heads/master@{#468276}