When creating a offline region, we've previously only requested the sprite image for the specified resolution. This lead to offline packs not being usable on devices that have a different pixel ratio. We're now requesting both 1x and 2x sprites. Some devices use even higher or fractional pixel ratios. However, we only ever use 1x and 2x sprite images in our requests.
Before this change, we've tried to open the database in read/write, but not create mode. In situations where the database didn't exist yet, this logged an error to the console, and we proceeded to opening it again in read/write/create mode, which actually created the file. The reason we did this is so that we could detect really old caching databases from January 2016 in case a developer upgraded from an older SDK (iOS v3.1 and earlier, Android v3.2 and earlier) that didn't have https://github.com/mapbox/mapbox-gl-native/pull/3715 yet.
However, these error messages, while innocent, look scary in the console and some users suspect that it's a bug. This change opens the file directly in read/write/create mode, omitting the first failed attempt. To handle old cache databases, we're now deleting the `http_cache` table, which was the only table in those old databases, and create the new schema, rather than deleting the entire file and recreating the Database object. In most scenarios, this will lead to one fewer opening attempt, while the database migration will continue to work for the few users who upgrade all the way from a January 2016 SDK.
Additionally, this fixes a mismatch between the Qt and non-Qt implementation: Qt doesn't support opening a file in read/write mode without the create flag. This means that we've seen a different control flow on Qt compared to the non-Qt implementation when opening a database.
Unfortuntely, it's difficult to avoid all exceptions, because sqlite3_open_v2 does not reliably return SQLITE_NOTADB if the file is not a database. However, this should avoid cases where developers misinterpret the SQLITE_CANTOPEN exception as a crash, which is the common case.
This adds a `convertJSON` template, to be used like:
Error error
optional<Foo> foo = convertJSON<Foo>(string, error);
Internally, it parses the string with RapidJSON and then calls `convert<Foo>(parsed, error)`.
While here, rationalize GeoJSON converters and fix error handling for Tileset conversion in OfflineDownload.
Continue to use uint32 as glyph ID to maintain Glyph PBF, even though we're only using 16 bits of that uint32.
Use std::codecvt instead of boost::unicode_iterator for UTF8->UTF16 conversions.
