Add/use display.drawImage which is viewport relative.

Fixes https://github.com/kanaka/noVNC/issues/163

When using an encoding with raw images (tight, tightPNG) we need to
draw those image relative to the viewport so that clipping works when
the viewport isn't at 0, 0.

include/display.js

@@ -577,6 +577,12 @@ that.blitStringImage = function(str, x, y) {
     img.src = str;
 };
 
+// Wrap ctx.drawImage but relative to viewport
+that.drawImage = function(img, x, y) {
+    c_ctx.drawImage(img, x - viewport.x, y - viewport.y);
+};
+
+
 that.changeCursor = function(pixels, mask, hotx, hoty, w, h) {
     if (conf.cursor_uri === false) {
         Util.Warn("changeCursor called but no cursor data URI support");

include/rfb.js

@@ -1562,7 +1562,7 @@ scan_tight_imgQ = function() {
             } else if (data.type === 'rgb') {
                 display.blitRgbImage(data.x, data.y, data.width, data.height, data.img.data, 0);
             } else {
-                ctx.drawImage(data.img, data.x, data.y);
+                display.drawImage(data.img, data.x, data.y);
             }
         }
         setTimeout(scan_tight_imgQ, scan_imgQ_rate);