fix Text.create()
use 'simple' line shader in BasicOverlay
release TextItems to pool
set up mBitmaps so that TextureItems can be created on initialization in Main Thread
make Slot public for testing
- extracted MapEventLayer from TouchHandler
- rename OverlayManager to LayerManager
- move event-dispatching from TouchHandler to LayerManager
Triangle: no need to shift vertices, just update segment indices to first occurence