font-patcher: Fix 2 cell glyphs with overlap

[why]
Glyphs that shall end up in two cells with xy scaling and overlap (for
example E0C0, E0C2) end up being a bit too wide.
In fact the width is the 2 cell width and 2 overlap width.

[how]
Apply the overlap (which is 'percent' factor) not on the concrete glyph
width, which is 2*cellwidth in these cases, but base it on the
one-cell-width instead.

Signed-off-by: Fini Jastrow <ulf.fini.jastrow@desy.de>

font-patcher

@@ -1373,7 +1373,7 @@ class font_patcher:
 
         target_width = self.font_dim['width'] * self.get_target_width(stretch)
         if overlap:
-            target_width *= 1.0 + overlap
+            target_width += self.font_dim['width'] * overlap
         scale_ratio_x = target_width / sym_dim['width']
 
         # font_dim['height'] represents total line height, keep our symbols sized based upon font's em