Add tilePixelRatio option to ol.source.WMTS

This PR adds a tilePixelRatio option to ol.source.WMTS. This option can be used with WMTS services that send larger tiles (512x512) than advertized (256x256), for high quality rendering on hidpi/retina screens. The PR includes an example (from @ahocevar) demonstrating the use of that option.

Meant to replace #2194.
Minor, but you use `256px` (no space) and `512 px` (space). Could be made consistent.