Merge remote branch 'upstream/master'
MP: incremental update for terrain layers for add/remove/etc.
Update DateTime.cpp

Without string.h memset is not declared
Merge pull request #357 from mucisk/patch-1

Update DateTime.cpp
Call osg::Program::releaseGLObjects from within VirtualProgram::releaseGLObjects
Incremental terrain update in MP: almost done, just need a solution for elevation visiblity toggle
Merge pull request #358 from chrisstankevitz/releaseglobjects

Call osg::Program::releaseGLObjects from within VirtualProgram::releaseG...
Incremental update: make it work with varying LODs and upsampled tiles
Incremental update: add rejection of out-of-date arrivals
TerrainLayer: added automatic cache invalidation based on TileSource last-modified-time; implemented for TMS only so far
Fix: TECHNIQUE_SCENE was not working becuase MP engine was not firing Terrain events for upsampled tiles
Fix: #363 tracks demo not working properly
Fix: Draping and Clamping techniques were using too much memory; had to set the camera's implicit render/resolve masks to zero to prevent OSG from automatically allocating unneeded buffers.
Merge branch 'master' into topic-incremental-update
Controls work
Updated feature editing tools to use FeatureNode instead of FeatureModelGraph

Fixed auto clamping in Draggers, the _mapNode variable was being set in
the constructor and therefore was skipping the oldMapNode check in
setMapNode which sets up the clamping.
Fix controls issues with rendering order and font bins
Fix #351 - there was no PolygonSymbol present
Merge branch 'master' of github.com:gwaldron/osgearth
MP: added an expiration collector to fix a mem leak