Fix drawing of B_GRAY1 bitmaps.

- Colors were reversed
- Padding was not handled properly because of a roundeing error

Add a test that shows the issue (behavior confirmed against BeOS)

Change-Id: I4c6e954fb6bdab92ad4e0e96897e78b26eb4727b
Reviewed-on: https://review.haiku-os.org/c/1025
Reviewed-by: Stephan Aßmus <superstippi@gmx.de>