Added the mbtiles file source for rendering vector tiles from file stored locally on the device.

include/mbgl/storage/file_source.hpp

@@ -22,6 +22,7 @@ enum FileSourceType : uint8_t {
     Database,
     FileSystem,
     Network,
+    Mbtiles,
     // Resource loader acts as a proxy and has logic
     // for request delegation to Asset, Cache, and other
     // file sources.

include/mbgl/util/constants.hpp

@@ -62,6 +62,7 @@ constexpr int DEFAULT_RATE_LIMIT_TIMEOUT = 5;
 constexpr const char* API_BASE_URL = "https://api.mapbox.com";
 constexpr const char* ASSET_PROTOCOL = "asset://";
 constexpr const char* FILE_PROTOCOL = "file://";
+constexpr const char* MBTILES_PROTOCOL = "mbtiles://";
 constexpr uint32_t DEFAULT_MAXIMUM_CONCURRENT_REQUESTS = 20;
 
 constexpr uint8_t TERRAIN_RGB_MAXZOOM = 15;