[vector-api] High dpi devices support

WORK IN PROGRESS

todo:
  - [x] scale vector line width by `devicePixelRatio`
  - [ ] scale `ol.shape` by `devicePixelRatio`
  - [x] use `devicePixelRatio` in simplify (tolerance)
  - [x] `ol.source.ImageWMS` support
  - [x] `ol.source.MapGuide` support
  - [ ] `ol.source.TileJSON` support (https://www.mapbox.com/developers/api/#Image.quality.&.scale)
  - [ ] webgl and dom support