Explicitly default-construct array

The docs seem to imply this is automatic when the array contains a
class-type, which osg::ref_ptr is, but I got a crash log that doesn't
make sense if that's true.

components/sceneutil/mwshadowtechnique.cpp

@@ -782,13 +782,15 @@ void MWShadowTechnique::ViewDependentData::releaseGLObjects(osg::State* state) c
 MWShadowTechnique::MWShadowTechnique():
     ShadowTechnique(),
     _enableShadows(false),
-    _debugHud(nullptr)
+    _debugHud(nullptr),
+    _castingPrograms{ nullptr, nullptr, nullptr, nullptr, nullptr, nullptr, nullptr, nullptr }
 {
     _shadowRecievingPlaceholderStateSet = new osg::StateSet;
 }
 
 MWShadowTechnique::MWShadowTechnique(const MWShadowTechnique& vdsm, const osg::CopyOp& copyop):
     ShadowTechnique(vdsm,copyop)
+    , _castingPrograms(vdsm._castingPrograms)
 {
     _shadowRecievingPlaceholderStateSet = new osg::StateSet;
     _enableShadows = vdsm._enableShadows;