Fixed warnings reported by gcc-9.3.0

Note from glebm@:

Back-ported from e0d5e4b0ff

Identical except for the changes to `MatrixTemplate`, which does not
exist in 3.6

include/osg/BoundingSphere

@@ -63,6 +63,13 @@ class BoundingSphereImpl
           * otherwise. */
         inline bool valid() const { return _radius>=0.0; }
 
+        inline BoundingSphereImpl& operator = (const BoundingSphereImpl& rhs)
+        {
+            _center = rhs._center;
+            _radius = rhs._radius;
+            return *this;
+        }
+
         inline bool operator == (const BoundingSphereImpl& rhs) const { return _center==rhs._center && _radius==rhs._radius; }
         inline bool operator != (const BoundingSphereImpl& rhs) const { return _center!=rhs._center || _radius!=rhs._radius; }
 

include/osg/GLExtensions

@@ -149,6 +149,15 @@ class VertexAttribAlias
             _osgName(osgName),
             _declaration(declaration) {}
 
+        VertexAttribAlias& operator = (const VertexAttribAlias& rhs)
+        {
+            _location = rhs._location;
+            _glName = rhs._glName;
+            _osgName = rhs._osgName;
+            _declaration = rhs._declaration;
+            return *this;
+        }
+
         GLuint      _location;
         std::string _glName;
         std::string _osgName;

include/osg/Quat

@@ -53,6 +53,14 @@ class OSG_EXPORT Quat
             _v[3]=w;
         }
 
+        inline Quat( const Quat& rhs )
+        {
+            _v[0]=rhs._v[0];
+            _v[1]=rhs._v[1];
+            _v[2]=rhs._v[2];
+            _v[3]=rhs._v[3];
+        }
+
         inline Quat( const Vec4f& v )
         {
             _v[0]=v.x();