[designspace] Hack to read avar2 mapping

This just uses list of list of dictionaries. Should use proper objects.

#3049

https://github.com/harfbuzz/boring-expansion-spec/blob/main/avar2-in-designspace.md

Lib/fontTools/designspaceLib/__init__.py

@@ -2005,6 +2005,24 @@ def readAxes(self):
             self.documentObject.axes.append(axisObject)
             self.axisDefaults[axisObject.name] = axisObject.default
 
+        mappingElement = self.root.find(".axes/mapping")
+        self.documentObject.mapping = []
+        if mappingElement is not None:
+            for regionElement in mappingElement.findall("region"):
+                inputElement = regionElement.find("input")
+                outputElement = regionElement.find("output")
+                inputLoc = {}
+                outputLoc = {}
+                for dimElement in inputElement.findall(".dimension"):
+                    tag = dimElement.attrib["tag"]
+                    value = float(dimElement.attrib["xvalue"])
+                    inputLoc[tag] = value
+                for dimElement in outputElement.findall(".dimension"):
+                    tag = dimElement.attrib["tag"]
+                    value = float(dimElement.attrib["xvalue"])
+                    outputLoc[tag] = value
+                self.documentObject.mapping.append([inputLoc, outputLoc])
+
     def readAxisLabel(self, element: ET.Element):
         xml_attrs = {
             "userminimum",