[varLib] Add mutator, which instantiates a variation font

Run, eg:

$ python mutator.py ./NotoSansArabic-GX.ttf wght=140 wdth=85

API to be cleaned up.

Lib/fontTools/varLib/mutator.py

@@ -0,0 +1,88 @@
+"""
+Module for dealing with 'gvar'-style font variations, also known as run-time
+interpolation.
+
+The ideas here are very similar to MutatorMath.  There is even code to read
+MutatorMath .designspace files.
+
+For now, if you run this file on a designspace file, it tries to find
+ttf-interpolatable files for the masters and build a GX variation font from
+them.  Such ttf-interpolatable and designspace files can be generated from
+a Glyphs source, eg., using noto-source as an example:
+
+  $ fontmake -o ttf-interpolatable -g NotoSansArabic-MM.glyphs
+
+Then you can make a GX font this way:
+
+  $ python fonttools/Lib/fontTools/varLib/__init__.py master_ufo/NotoSansArabic.designspace
+
+API *will* change in near future.
+"""
+from __future__ import print_function, division, absolute_import
+from fontTools.misc.py23 import *
+from fontTools.ttLib import TTFont
+from fontTools.ttLib.tables._g_l_y_f import GlyphCoordinates
+from fontTools.varLib import VariationModel, supportScalar, _GetCoordinates, _SetCoordinates
+import os.path
+
+def main(args=None):
+
+	import sys
+	if args is None:
+		args = sys.argv[1:]
+
+	varfilename = args[0]
+	locargs = args[1:]
+	outfile = os.path.splitext(varfilename)[0] + '-instance.ttf'
+
+	loc = {}
+	for arg in locargs:
+		tag,valstr = arg.split('=')
+		assert len(tag) == 4
+		loc[tag] = float(valstr)
+	print("Location:", loc)
+
+	print("Loading GX font")
+	varfont = TTFont(varfilename)
+
+	fvar = varfont['fvar']
+	for axis in fvar.axes:
+		lower, default, upper = axis.minValue, axis.defaultValue, axis.maxValue
+		v = loc.get(axis.axisTag, default)
+		if v < lower: v = lower
+		if v > upper: v = upper
+		if v == default:
+			v = 0
+		elif v < default:
+			v = (v - default) / (default - lower)
+		else:
+			v = (v - default) / (upper - default)
+		loc[axis.axisTag] = v
+	# Location is normalized now
+	print("Normalized location:", loc)
+
+	gvar = varfont['gvar']
+	for glyphname,variations in gvar.variations.items():
+		coordinates,_ = _GetCoordinates(varfont, glyphname)
+		for var in variations:
+			scalar = supportScalar(loc, var.axes)
+			if not scalar: continue
+			coordinates += GlyphCoordinates(var.coordinates) * scalar
+		_SetCoordinates(varfont, glyphname, coordinates)
+
+	print("Removing GX tables")
+	for tag in ('fvar','avar','gvar'):
+		if tag in varfont:
+			del varfont[tag]
+
+	print("Saving instance font", outfile)
+	varfont.save(outfile)
+
+
+if __name__ == "__main__":
+	import sys
+	if len(sys.argv) > 1:
+		main()
+		#sys.exit(0)
+	import doctest, sys
+	sys.exit(doctest.testmod().failed)