art_render_mask rolled into libart_lgpl

[Darin Adler <darin bentspoon com>]

After discussing it with Anders, I moved the art_render_mask function from
librsvg to libart_lgpl. This was always part of libart, and was in librsvg
for historical reasons. At the moment, the only use of the function within
Gnome is in librsvg (for obvious reasons).

I also fixed the libart_lgpl headers so they include things they depend on.

Please let me know if there's a problem with either of these changes.

    -- Darin