[instancer] Start avar2 subsetting

Just some code for now to get min/max delta of a
varIdx from a varStore...

Lib/fontTools/varLib/instancer/avar.py

@@ -0,0 +1,84 @@
+from fontTools.varLib.varStore import VarStoreInstancer, NO_VARIATION_INDEX
+from fontTools.ttLib.tables.otTables import VarStore
+from fontTools.ttLib.tables._f_v_a_r import Axis
+
+
+def VarStore_getExtremes(self, varIdx, nullAxes=set(), cache=None):
+
+    if varIdx == NO_VARIATION_INDEX:
+        return 0, 0
+
+    if cache is None:
+        cache = {}
+
+    key = (varIdx, frozenset(nullAxes))
+    if key in cache:
+        return cache[key]
+
+    regionList = self.VarRegionList
+    fvar_axes = []
+    for i in range(regionList.RegionAxisCount):
+        axis = Axis()
+        axis.axisTag = str(i)
+        axis.minValue = -1.0
+        axis.defaultValue = 0.0
+        axis.maxValue = 1.0
+        fvar_axes.append(axis)
+
+    major = varIdx >> 16
+    minor = varIdx & 0xFFFF
+    varData = self.VarData[major]
+    regionIndices = varData.VarRegionIndex
+
+    minV = 0
+    maxV = 0
+    for regionIndex in regionIndices:
+        location = {}
+        region = regionList.Region[regionIndex]
+        skip = False
+        thisAxes = set()
+        for i, regionAxis in enumerate(region.VarRegionAxis):
+            peak = regionAxis.PeakCoord
+            if peak == 0:
+                continue
+            if i in nullAxes:
+                skip = True
+                break
+            thisAxes.add(i)
+            location[str(i)] = peak
+        if skip:
+            continue
+        instancer = VarStoreInstancer(varStore, fvar_axes, location)
+        v = instancer[varIdx]
+
+        assert thisAxes, "Empty region in VarStore!"
+        minOther, maxOther = self.getExtremes(varIdx, nullAxes | thisAxes, cache)
+
+        minV = min(minV, v + minOther)
+        maxV = max(maxV, v + maxOther)
+
+    cache[key] = (minV, maxV)
+
+    return minV, maxV
+
+
+VarStore.getExtremes = VarStore_getExtremes
+
+
+if __name__ == "__main__":
+    import sys
+    from fontTools.ttLib import TTFont
+
+    font = TTFont(sys.argv[1])
+    fvar = font["fvar"]
+    avar = font["avar"]
+
+    varIdxMap = avar.table.VarIdxMap
+    varStore = avar.table.VarStore
+
+    for axisIdx, axis in enumerate(fvar.axes):
+        varIdx = axisIdx
+        if varIdxMap is not None:
+            varIdx = varIdxMap[varIdx]
+        minV, maxV = varStore.getExtremes(varIdx)
+        print(axis.axisTag, (minV / 16384, maxV / 16384))