Add missing call to markDirty() in MapNode::updateFBO()

Qml map view was not being updated correctly when user dragged the map,
position changed logically but not visually. Adding a call to markDirty()
solves the problem.

Change-Id: Ice6a8607f852d9778db52f0958fbd79efff1ccce
Reviewed-by: Aaron McCarthy <mccarthy.aaron@gmail.com>