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>

include/osgPresentation/SlideShowConstructor

@@ -267,7 +267,8 @@ public:
             region_in_pixel_coords(false),
             alphaValue(1.0),
             cutoffValue(0.1),
-            sampleDensityValue(0.005)
+            sampleDensityValue(0.005),
+            sampleDensityWhenMovingValue(0.02)
         {
             region[0] = region[1] = region[2] = 0.0f;
             region[3] = region[4] = region[5] = 1.0f;
@@ -282,6 +283,7 @@ public:
         float                                   alphaValue;
         float                                   cutoffValue;
         float                                   sampleDensityValue;
+        float                                   sampleDensityWhenMovingValue;
     };