[core] Calculate size of an ambient cache without offline region's resources

[alexshalamov]

Resources that belong to an offline region, should not contribute to the amount of space available in the ambient cache.

Fixes: #4411
Fixes: https://github.com/mapbox/mapbox-gl-native-team/issues/212

[alexshalamov]

@tobrun @julianrex Documentation for darwin and Android has to be updated. Do you want to do it yourself in separate PR?

Also, this is an API break, since the old behavior, is changed, and some apps may have a workarounds that resizes ambient cache every-time offline pack is downloaded. Not sure if worth making semver release.

[pozdnyakov]

few nits

[tmpsantos]

The only concern I have about this approach is the ambient cache size reduction is precisely measure using the delta of number of pages but the growth is taking into consideration only the data size.

Meaning that if we add and remove the same entry, the added size is always going to be smaller than the removed size, and in the long run the cache size will be zero.

[alexshalamov]

@tmpsantos Changed code a bit to get more precise ambient cache growth size, ptal.

[chloekraw]

Also, this is an API break, since the old behavior, is changed, and some apps may have a workarounds that resizes ambient cache every-time offline pack is downloaded. Not sure if worth making semver release.

In my opinion, if we flag this change clearly at the top of the changelog under a "Major improvements" section, this would be acceptable in a minor release. cc @mapbox/maps-android @mapbox/maps-ios

@alexshalamov could you draft a changelog entry and I'll think about it and review? Thanks for picking this longstanding problem up!

[alexshalamov]

@julianrex @1ec5 @tobrun we are planning to release this fix in a next release (maps-v1.4.0)

[pozdnyakov]

lgtm % nits