The big NULL replacement party part 1.

This is meant to be a safer alternative since
NULL can often be 0. Let's not rely on that.

And yes, I know this is a lot of files. This is
a safer thing to do in big commits vs for loops.

src/ActorMultiTexture.cpp

@@ -65,7 +65,7 @@ void ActorMultiTexture::ClearTextures()
 
 int ActorMultiTexture::AddTexture( RageTexture *pTexture )
 {
-	ASSERT( pTexture != NULL );
+	ASSERT( pTexture != nullptr );
 	LOG->Trace( "ActorMultiTexture::AddTexture( %s )", pTexture->GetID().filename.c_str() );
 
 	m_aTextureUnits.push_back( TextureUnitState() );