Added support for new SampleDensityWhenMovingProperty into .p3d format, with it default to 0.02.

One can override this value via the sampleDensityWhenMoving="value" property in the volume tag, i.e.

         <volume sampleDensityWhenMoving="0.01">CardiacCT</volume>

To switch the feature off set the value to 0, i.e.

         <volume sampleDensityWhenMoving="0">CardiacCT</volume>

src/osgPlugins/p3d/ReaderWriterP3D.cpp

@@ -1004,6 +1004,7 @@ void ReaderWriterP3DXML::parseVolume(osgPresentation::SlideShowConstructor& cons
     if (getProperty(cur, "cutoff", volumeData.cutoffValue)) {}
     if (getProperty(cur, "region", 6, volumeData.region)) {}
     if (getProperty(cur, "sampleDensity", volumeData.sampleDensityValue)) {}
+    if (getProperty(cur, "sampleDensityWhenMoving", volumeData.sampleDensityWhenMovingValue)) {}
 
     // check for any transfer function required
     std::string transferFunctionFile;