Improve font patcher

For some reason the font patcher doesn't work well unless the target
font has an em size of 2048 while the font is being patched. This patch
also improves font patching in general, due to the more correct
calculation of font metrics.

Refs #14.

powerline/fontpatcher/fontpatcher.py

@@ -37,6 +37,9 @@ class FontPatcher(object):
 		for target_font in self.target_fonts:
 			source_font = self.source_font
 
+			target_font_em_original = target_font.em
+			target_font.em = 2048
+
 			target_font.encoding = 'ISO10646'
 
 			# Rename font
@@ -74,8 +77,7 @@ class FontPatcher(object):
 
 			# Find source and target midpoints for translating
 			x_diff = target_bb[0] - source_bb[0]
-			y_diff = (target_bb[3] + target_bb[1]) - (source_bb[3] + source_bb[1])
-
+			y_diff = target_bb[1] - source_bb[1]
 			translate = psMat.translate(x_diff, y_diff)
 
 			transform = psMat.compose(scale, translate)
@@ -97,6 +99,8 @@ class FontPatcher(object):
 				# Transform the glyph
 				target_font.transform(transform)
 
+			target_font.em = target_font_em_original
+
 			# Generate patched font
 			target_font.generate('{0}.otf'.format(target_font.fullname))