models: convert delta array to float if overflows signed short integers

We should check other places where such overflows may occur and do the same.
Or do maybe it in GlyphCoordinates class.

Lib/fontTools/varLib/models.py

@@ -299,7 +299,12 @@ class VariationModel(object):
 		for i,weights in enumerate(self.deltaWeights):
 			delta = masterValues[mapping[i]]
 			for j,weight in weights.items():
-				delta -= out[j] * weight
+				try:
+					delta -= out[j] * weight
+				except OverflowError:
+					# if it doesn't fit signed shorts, retry with doubles
+					delta._ensureFloat()
+					delta -= out[j] * weight
 			out.append(delta)
 		return out