Replace ImageLayer fallback mechanisms with simple Tile resuse in the terrain engine
Heightfield fallback removed from elevation layer; replaced in terrain engine
Enabled tile cancelation in MP
cleanup
Remove deprecated elevation code
reset MP stats on tile cancelation
ocean/terrain: update to new layer api
ElevationLayerVector: only load heightfields on demand
MP driver: better profiling data
add --activity flag to track pager activity
add activity mon
glsl checks, ocean fix, geodata
Rewrote ElevationQuery to avoid fallbacks

ElevationQuery now tries to get values from each elevation layer one at
a time vs compositing them all together.  This avoids doing work
grabbing tiles that you don't really need for a query.
Merge branch 'topic-fallback' of github.com:gwaldron/osgearth into topic-fallback
Another attempt at ElevationQuery.  this time using populateHeightField.
Merge branch 'topic-fallback' of github.com:gwaldron/osgearth into topic-fallback
Changed populateHeightField to use the origin and interval values from the key instead of the HeightField.
Merge branch 'topic-fallback' of github.com:gwaldron/osgearth into topic-fallback
Elevation: add 'mapped resolution' heightfield generation
Add support for tilekey resolution mapping