Display extrapolated tiles on nearby zooms.

This issue needs more words of explanation, so that others can understand the issue and contribute.

Chris.

When using OSN, it firstly displays tiles that were previously stored in the cache (sdcard).
If not, it tries to download the tile over internet.
The problem is when connection is flacky or inexistant, the map stays blank.

So, when requesting tiles over internet,
if tiles at zoom+1 or zoom-1 are available we should use them and firstly display them using extrapolation, even it's not very accurate, it's obviously better than a blank one.
Then, if tiles are correctly downloaded we could display them.

This is available in osmdroid, so I am actually going to port it over this time :)

Thanks for the rapid updates, guys. Any thoughts on the target release?

Chris.