Merge pull request #1 from pauldendulk/master

Merge from pauldendulk
Improved tile retrieval during rendering

When performing recursive searches for tiles to render, I converted the
resolution dictionary to a sorted list and used indices to move up to
lower resolutions instead of performing dictionary look ups each time.
This greatly improves performance when tiles are found right away
resulting in multiple recursive calls.
adding RenderGetStrategyTests
Merge branch 'scottdewald-master'
Merge branch 'master' of https://github.com/pauldendulk/Mapsui