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/Texture.cpp

@@ -2258,6 +2258,7 @@ Texture::Extensions::Extensions(unsigned int contextID)
         OSG_INFO<<"Disabling _isNonPowerOfTwoTextureMipMappedSupported for GeForce FX hardware."<<std::endl;
     }
 
+    _maxTextureSize=0;
     glGetIntegerv(GL_MAX_TEXTURE_SIZE,&_maxTextureSize);
 
     char *ptr;
@@ -2274,6 +2275,7 @@ Texture::Extensions::Extensions(unsigned int contextID)
 
     if( _isMultiTexturingSupported )
     {
+       _numTextureUnits = 0;
        #if defined(OSG_GLES2_AVAILABLE) || defined(OSG_GL3_AVAILABLE)
            glGetIntegerv(GL_MAX_TEXTURE_IMAGE_UNITS,&_numTextureUnits);
        #else