* Consolidate code for determining the pixel ratio
* - Removed local param 'pixerRatio' from CanvasRenderer. Now only canvas.pixelRatio is used.
- consolidated ctx.transform() calls in Canvas
- Added/edited commenting (also TODO's)
- Added Canvas.getContext()
* CachedImage with preredendered zoom images; first working version.
* Fixes for missing images and usage brokenImage.
* Remove unused method
* Added height member of CachedImage, note about svg's