fix gcc warnings : base class ctor.

warning: base class ... should be explicitly initialized in the copy constructor
fix gcc warnings : -Wignored-qualifiers.

warning: type qualifiers ignored on function return type [-Wignored-qualifiers]
Merge pull request #515 from polch/gcc-extra-warnings

Gcc extra warnings
atlas: allow different X and Y sizes
TMS: add storeImage() support for the TMS tile source
conv: add support for elevation data
DrawInstanced: added an API to access instace positioning matrices
Registry: set a glyph margin on default font
Docs update
Add StateSetLOD
moved StateSetLOD into osgEarth namespace
gcc compile patch
Merge branch 'master' into topic-weather
Added elevation layer to weather demo