Add process feedback on tiles-export

[yohanboniface]

At the moment, the only feedback about the process is the number of tiles to be created by zoom.

We need something like an ETA.

[pnorman]

I have tried really hard to generate progress estimates for tile generation. Within a zoom is possible, but inter-zoom progress estimates suck, because progress rates are very zoom-dependent.

I wrote directions for a saved tile directory with avecado where I had a progress estimate, which reported like this

0: 1/1
1: 4/4
2: 16/16
3: 64/64
4: 256/256
5: 1024/1024
6: 4096/4096
7: 16384/16384
8: 65536/65536
9: 262144/262144
10: 969486/1048576

When I used the higher zoom rates to estimate the total time, I was off by more than an order of magnitude. With 60% of the tiles being the last zoom and nearly all being the last two zooms, that's the only rate that matters, and it is common for rates to vary by an order of magnitude or more between zooms.

If you use an algorithm which traverses zooms before geographic position, then you might get valid rates.