font-patcher: Respect gap values in source font

[why]
We just ignore specified gaps in the source fonts (i.e. set them to
zero). This reduces the line spacing in the patched font (because the
gap is missing).

[how]
Distribute the gap INTO the cell, so that we can work with zero gap (we
need that for the powerline glyphs), and still keeping the powerline
glyphs centered about the regular glyphs AND keeping the total line
spacing.

Idea-by: Tushar Singh <tusharvickey1999@gmail.com>

Fixes: #850

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

font-patcher

@@ -814,11 +814,6 @@ class font_patcher:
             self.sourceFont.hhea_ascent = self.sourceFont.os2_winascent
             self.sourceFont.hhea_descent = -self.sourceFont.os2_windescent
 
-        # Line gap add extra space on the bottom of the line which
-        # doesn't allow the powerline glyphs to fill the entire line.
-        self.sourceFont.hhea_linegap = 0
-        self.sourceFont.os2_typolinegap = 0
-
     def get_essential_references(self):
         """Find glyphs that are needed for the basic glyphs"""
         # Sometimes basic glyphs are constructed from multiple other glyphs.
@@ -845,6 +840,23 @@ class font_patcher:
             self.font_dim['ymin'] = self.sourceFont.os2_typodescent
             self.font_dim['ymax'] = self.sourceFont.os2_typoascent
 
+        # Line gap add extra space on the bottom of the line which
+        # doesn't allow the powerline glyphs to fill the entire line.
+        # Put half of the gap into the 'cell', each top and bottom
+        gap = max(self.sourceFont.hhea_linegap, self.sourceFont.os2_typolinegap) # TODO probably wrong
+        if self.sourceFont.os2_use_typo_metrics:
+            gap = self.sourceFont.os2_typolinegap
+        self.sourceFont.hhea_linegap = 0
+        self.sourceFont.os2_typolinegap = 0
+        if gap > 0:
+            gap_top = int(gap / 2)
+            gap_bottom = gap - gap_top
+            self.font_dim['ymin'] -= gap_bottom
+            self.font_dim['ymax'] += gap_top
+            self.sourceFont.os2_typoascent = self.sourceFont.os2_typoascent + gap_top
+            self.sourceFont.os2_typodescent = self.sourceFont.os2_typodescent - gap_bottom
+            # TODO Check what to do with win and hhea values
+
         # Find the biggest char width
         # Ignore the y-values, os2_winXXXXX values set above are used for line height
         #