* api change - all drawing functions now use draw mode, foreground and
background shades set globally by separate functions
* There are now 4 draw modes for all drawing functions, no more separate
functions for inverse drawing
* Significant speedup of 1-bit bitmap (and font) drawing (2..3 times)
* Some more speed tweaks
* Additional functions for horizontal and vertical lines
* Copied describing comments to the header file for easier reference
* The safety net against an uninitialized grayscale buffer is gone
git-svn-id: svn://svn.rockbox.org/rockbox/trunk@4711 a1c6a512-1295-4272-9138-f99709370657
