From David Fries, "Fix remote X11 crash querying GL_NUM_EXTENSIONS

In osg::isGLExtensionOrVersionSupported in src/osg/GLExtensions.cpp when
using indirect X11 rendering,
glGetIntegerv( GL_NUM_EXTENSIONS, &numExt );
is leaving numExt uninitilized causing the following glGetStringi to
return NULL when the extension number isn't present.  Passing NULL to
std::string() then crashes.  This is with the following nVidia driver.
OpenGL version string: 3.3.0 NVIDIA 256.35

I went ahead and initialized some of the other variables before
glGetInitegerv in other files as well.  I don't know for sure
which ones can fail, so I don't know which are strictly required.
"

src/osg/Texture3D.cpp

@@ -606,6 +606,7 @@ void Texture3D::Extensions::setupGLExtensions(unsigned int contextID)
     if (_isTexture3DFast) _isTexture3DSupported = true;
     else _isTexture3DSupported = strncmp((const char*)glGetString(GL_VERSION),"1.2",3)>=0;
     
+    _maxTexture3DSize = 0;
     glGetIntegerv(GL_MAX_3D_TEXTURE_SIZE, &_maxTexture3DSize);
 
     setGLExtensionFuncPtr(glTexImage3D,"glTexImage3D","glTexImage3DEXT");