Properly store bit-depth for bitmaps when generating ttc.

Before this change it is impossible to create a ttc with embedded
monochrome bitmaps from a python script, as the bit depth for such
bitmaps was set to 0. Before this scripts like

  font.bitmapSizes = ((12,),)
  font.regenBitmaps((12,))
  font.generateTtc("font.ttc", None, bitmap_type="ttf", layer=1)

would result in an error message like

  The font database does not contain a bitmap of size 12 and depth 0