Making tile scaling work with the Zoomify layer

Because zoomify tiles have different sizes, we also need to store the
standard tile size on the backbuffer. The Zoomify layer gets an override
for the createBackBuffer method to make sure that the native image size
is used.
Fixing superclass constructor call
Merge pull request #1057 from ahocevar/zoomify-backbuffer

Making tile scaling work with the Zoomify layer