Respect server's tile pixel ratio in TileImageSource and add wmts-hidpi example

A pixelRatio defined as `ol.source.TileImage` option specifies the pixel
ratio provided by the server. With this change, HiDPI tile sources are
finally possible. The WMTS example is a proof of concept.
That's the part I do not like. I'll see if I can find a better way. So far I haven't been able to think of a better way.