ddddoooouuuubbbblllleeeebbbbuuuuffffffffeeeerrrr sets the display mode to double buffer mode. It does not
take effect until ggggccccoooonnnnffffiiiigggg is called. In double buffer mode, the
bitplanes are partitioned into two groups, the front bitplanes and the
back bitplanes. Double buffer mode displays only the front bitplanes.
Drawing routines normally update only the back bitplanes; ffffrrrroooonnnnttttbbbbuuuuffffffffeeeerrrr and
bbbbaaaacccckkkkbbbbuuuuffffffffeeeerrrr can override the default.
In double buffer mode, ggggccccoooonnnnffffiiiigggg calls ffffrrrroooonnnnttttbbbbuuuuffffffffeeeerrrr((((OOOOFFFFFFFF)))) and bbbbaaaacccckkkkbbbbuuuuffffffffeeeerrrr((((OOOONNNN))))....