[vector-api] HiDPI support for tile layers

This PR adds support for HiDPI tile layers. HiDPI support is achieved by requesting a tile with more pixels (e.g. 512x512 pixels instead of 256x256 when the pixel ratio is 2).

Right now, `ol.source.TileWMS` when configured the appropriate `serverType` is the only source that is capable of generating such tiles.