add low res background image capability

[jgastonraoul]

It would be a great idea to display a low res image background while tiles are loading in order not to loose a "visible reference".
It should work like the downsample in the previous version but with the same image for all zoom levels.

[moagrius]

sounds like I should probably put that back in. I'll add it as a public method and let the dev worry about performance, next update. For now, you can try the hack here: #8

[moagrius]

closing this as the task has been assigned to me in #14