httpmsg screen drawing, use new ClearDontDraw

Author: addshore

src/services/httpmsg/html.toit

@@ -329,8 +329,7 @@ generate-screen-html -> string:
                     pixels: (endRow - startRow + 1) * gridWidth,
                     cArrayOutput: cArray.join(','),
                 }
-                const isLastRow = endRow === gridHeight - 1;
-                box.msgBytes = box2msgb(box, pageId, isLastRow);
+                box.msgBytes = box2msgb(box, pageId, (startRow === 0), (endRow === gridHeight - 1));
                 exportBoxes.push(box);
                 startRow = endRow + 1;
             }
@@ -359,7 +358,7 @@ generate-screen-html -> string:
         submitMulti(toSend);
     }
 
-    function box2msgb(box, pageId, isLastMsg= true) {
+    function box2msgb(box, pageId, isFirst=true, isLast=true) {
         const ui16le = (num) => {
             return [num & 0xff, (num >> 8) & 0xff];
         };
@@ -377,7 +376,9 @@ generate-screen-html -> string:
         d.set(9, [box.exportSizeX]);
         d.set(10, [box.exportSizeY]);
         d.set(25, box.cArrayOutput.split(',').map(byte => parseInt(byte, 16)));
-        if(isLastMsg) {
+        if(isFirst) {
+          d.set(6, [5]); // ClearDontDraw
+        } else if(isLast) {
           d.set(6, [4]); // FullRedrawWithoutClear
         } else {
           d.set(6, [3]); // BufferOnly