BGE - new read-only attribute in KX_GameObject python api (LOD level)

Added a new "current_lod_level" property to the python api of
KX_GameObject. The property returns the current lod level of the game
object. The purpose of the property is activate logic routines only when
an object is at a certain lod-distance from the camera, avoiding to
separately recomputing the same distance in the logic script. Usage in
python script might look like:

owner = bge.logic.getCurrentController().owner
lod_level = owner.currentLodLevel
if lod_level == 0: ...do something
else: ... object might be too distant

Reviewers: dfelinto, kupoman, moguri

Reviewed By: kupoman, moguri

Subscribers: lordloki

Projects: #game_engine

Differential Revision: https://developer.blender.org/D978