[xmonad] Invisible cursor with XFT font in Prompt

[lithis]

I recently enabled XFT support in XMonadContrib. When using
a TrueType font, the cursor in prompts is invisible. It
seems to be printing the character under the cursor with
bgColor as both the forground and background (I was able to
see the the edge of an “f” under the cursor overwriting the
edge of a preceeding “d”, so I know that it is being drawn).

Also, the visual length of the string changes as the cursor
moves. While scrolling through the string, the portion after
the cursor wiggles left and right by a few pixels. When the
cursor reaches the beginning of the line, the length
shortens much more.