Linux: use only advances for calculating cell sizes

There are apparently monospace fonts whose characters are not actually
monospaced when rendered, for example, Liberation Mono. It has
characters that when rendered result in bitmaps wider than the advance.

So we use only the advance as that is what most linux software seems to
do. Fixes #47

kitty/fonts/freetype.py

@@ -34,7 +34,7 @@ def calc_cell_width(font, face):
         ch = chr(i)
         load_char(font, face, ch)
         m = face.glyph_metrics()
-        ans = max(ans, max(ceil_int(m.horiAdvance / 64), face.bitmap().width))
+        ans = max(ans, ceil_int(m.horiAdvance / 64))
     return ans