grid: add CLRLN and change _lf behavior

Instead of clearing the rest of the line on a _lf, it's simpler
to just clear any new line we're entering into.

blk/402

@@ -1,10 +1,11 @@
 : XYPOS [ GRID_MEM LITN ] ;
 : _cl* COLS LINES * ;
 : AT-XY ( x y -- ) LINES * + _cl* MOD XYPOS ! ;
+'? CLRLN NIP NOT [IF]
+: CLRLN ( ln -- ) COLS * DUP COLS + SWAP DO 0 I CELL! LOOP ;
+[THEN]
 : _lf
-    XYPOS @ BEGIN ( pos )
-        0 ( blank ) SWAP TUCK CELL!
-        1+ DUP COLS MOD NOT UNTIL
-    _cl* MOD XYPOS ! ;
+    XYPOS @ COLS / 1+ DUP CLRLN
+    COLS * _cl* MOD XYPOS ! ;
 : _bs 0 ( blank ) XYPOS @ TUCK CELL! ( pos ) 1-
     _cl* MOD XYPOS ! ;

blk/403

@@ -3,4 +3,4 @@
     DUP 0x0d = IF DROP _lf EXIT THEN
     0x20 - DUP 0< IF DROP EXIT THEN
     XYPOS @ CELL!
-    XYPOS @ 1+ _cl* MOD XYPOS ! ;
+    XYPOS @ 1+ DUP COLS MOD IF XYPOS ! ELSE _lf THEN ;

doc/protocol.txt

@@ -46,6 +46,9 @@ COLS      -- n     Number of columns in the device
 LINES     -- n     Number of lines in the device
 CELL!     g pos -- Set glyph at pos
 
+Optional:
+CLRLN     ln --    Clear line number ln.
+
 "pos" is a simple number (y * cols) + x. For example, if we
 have 40 columns per line, the position (x, y) (12, 10) is 412.
 
@@ -53,3 +56,7 @@ A glyph is ASCII-0x20. If the resulting glyph number exceeds the
 number of glyphs in the font, it's up to CELL! to ignore it.
 
 Glyph 0 is always blank.
+
+If CLRLN is not defined, the grid system uses multiple CELL!
+calls to clear it. On some devices, this is highly inefficient.
+Drivers for those devices should define CLRLN.