Used by the routine at FILL_BYTE_TO_LEFT.

Jumped to from FILL_BYTE_TO_LEFT when the fill has just moved into a new byte. The whole byte is painted in one go only when nothing in it could change the fill's course: all eight pixels are clear in the clean copy, and each neighbouring row is the same all along the byte -- all clear if a run is open there (the run just goes on), all set if none is (none can open). Then all eight are marked in the clean copy, the texture's byte is written to the screen whole, and the loop goes on from the byte's left-hand pixel; the return address into FILL_NEXT_PIXEL is dropped so as to go straight to its top, ready for the next byte. Otherwise it returns and the byte is done pixel by pixel. On a room's big plain areas this paints eight pixels for about the work of one.