resolve callout offset issues

[incanus]

Per #1506 (comment) we should resolve callout offset issues in master to avoid regressions.

#1492 (comment)

[incanus]

The perceived offset issues are actually a manifestation of #1488, which is really two problems: marker images can disappear, and marker images can be at a mismatch with the current style. When they are a mismatch with the current style (e.g. showing Streets' smaller markers on Emerald), the JSON & image don't match up and so the callout is placed wrong.

Repro in GL demo app:

1. Start with Streets
2. Add 100 annotations
3. Switch to Emerald
4. Marker imagery remains Streets
5. Tap marker and observe callout dislocation

Workaround:

1. Start with Streets
2. Switch to Emerald
3. Add 100 annotations
4. Marker imagery is Emerald
5. No callout problems.

#1488 adequately covers our bases.