Correct the setup of texture cache in QGeoTiledMap for multiple maps

Currently, QGeoTiledMapPrivate::changeViewportSize calls
m_cache->setMinTextureUsage(newSize); where newSize is calculated based on the size
of the viewport, to make sure that the the texture cache will have at least enough space for the size of the viewport.

The problem is that m_cache is shared among all maps provided by the same provider.
Hence the minTextureUsage should be the sum of all viewport sizes of all attached maps