Made osg::Quat support either float or double internal representation, defaulting to double.

Generalised the osgDB::Field so that its getFloat() method can be used with either doubles or
floats governed by the type passed in - this helps support either float/double
Quat and Matrix classes seemlessly.

src/osgPlugins/osg/TexMat.cpp

@@ -44,7 +44,7 @@ bool TexMat_readLocalData(Object& obj, Input& fr)
         {
             for(int j=0;j<4;++j)
             {
-                fr[k].getDouble(v);
+                fr[k].getFloat(v);
                 matrix(i,j)=v;
                 k++;
             }