Features
- Initial implementation of multi-source tile source ([764](../../pull/764))
- Added pixelmap annotation element ([767](../../pull/767), [776](../../pull/776))
Improvements
- Add more opacity support for image overlays ([761](../../pull/761))
- Make annotation schema more uniform ([763](../../pull/763))
- Improve TileSource class repr ([765](../../pull/765))
- Improve frame slider response with base quads ([771](../../pull/771))
- Default to nearest-neighbor scaling in lossless image conversion ([772](../../pull/772))
- Improve the import time ([775](../../pull/775), [777](../../pull/777))
Bug Fixes
- The tile iterator could return excess tiles with overlap ([773](../../pull/773))