Make it easier to work with tile ranges in custom source implementations

[ahocevar]

This is an alternative to #5506. Instead of exposing tile ranges, this pull request makes a tile grid's tile coordinates available by extent and zoom level.

I do not like the name of the new function though. Any suggestions anyone? @tschaub maybe, since you're in a naming mood today it seems?

Fixes #5502.

[ashchurova]

Do you have an eta? Thanks.

[ahocevar]

This needs to be reviewed by another trusted developer. Since this is not paid work, it might take a while. But it shouldn't be too hard to review this one.

[ahocevar]

@fredj, I cherry-picked your other commit from #5506, so this pull request replaces that one. A review would be much appreciated.

[fredj]

LGTM, thanks Andreas

[tschaub]

For what it's worth, I don't think method names need to repeat the arguments. So, instead of this:

grid.forEachTileCoordInExtentAndZ(extent, z, callback);

my preference would be this:

grid.forEachTileCoord(extent, z, callback);

[ahocevar]

Thanks @tschaub, this was the feedback I was hoping for :-). Please review #5512.