Flush mouseMove when initiating viewport dragging

We want to flush pending mouse moves before we initiate viewport
dragging.

Before this commit, there were scenarios where the _mouseButtonMask
would track a released button as being down.

core/rfb.js

@@ -1103,6 +1103,8 @@ export default class RFB extends EventTargetMixin {
                         this._viewportDragPos = {'x': pos.x, 'y': pos.y};
                         this._viewportHasMoved = false;
 
+                        this._flushMouseMoveTimer(pos.x, pos.y);
+
                         // Skip sending mouse events, instead save the current
                         // mouse mask so we can send it later.
                         this._mouseButtonMask = bmask;