Potential Memory Leak when using multiple MapNodes in the scene

[jasonbeverage]

If you add multiple .earth files to the scene the # of PagedLOD's stays at a reasonable # (~300) but it seems like memory isn't released.

 http://forum.osgearth.org/register-Paged-LOD-tp5077618p5081535.html

[jasonbeverage]

This seems to have something to do with the #define EXPLICIT_RELEASE_GL_OBJECTS 1 in VersionedTerrain?.cpp. If you comment this out then memory goes back down to normal.

[jasonbeverage]

The reason that this isn't working is b/c only 1 postDrawCallback
is being assigned to the camera b/c of the use of setPostDrawcallback.

So only 1 of the tilesToRelease lists are being cleared out and the

other one is not.

We'll have to come up with some composite callback to handle multiple terrains.

[jasonbeverage]

Fixed in r1133