Draws a line of L+1 pixels from (D,E) in one of eight directions (bits 0-2 of C). Bit 0 says whether the line is mostly vertical or mostly horizontal; bits 1 and 2 give the vertical and horizontal directions. The line moves one pixel along its main direction every step, and one pixel along the other direction every B steps, so lines have slopes of 1, 1/2, 1/3 and so on rather than arbitrary angles. Drawing stops early at the edge of the picture area.