make valgrind shut up about uninitialized values in depth images

This might hide a real bug, that the depth textures are used, before they
were ever acquired.
remove compiler warning
Added new sensor, the MultiLevelLaserRangeFinder

This is a sensor implementation for a MultiLevelLaserRangeFinder,
e.g. a Velodnye HDL-32. This implementation is based on RTT images.