Make map preview generation fast.

- Change Map.LoadMapTiles and Map.LoadResourceTiles to read the whole stream into memory before processing individual bytes. This removes the cost of significant overhead from repeated calls to ReadUInt8/16.
- Fix StreamExts.ReadAllBytes to actually read all bytes and improve the Peek and ReadUInt8 methods to avoid a pointless array allocation which is significant overhead.
- Simplify and improve the async generation in MapCache. Work is tasked to the thread pool and multiple tasks may render in parallel which allows all cores to be brought to bear.
- Remove significant UI jank caused by the map chooser by not rendering the placeholder widget. The maps render fast enough that it is no longer worthwhile and it was causing a lot of flushes which were the source of the jank.
- Trigger async generation for all maps when the chooser is loaded. This means in practice all previews will be ready by the time the user begins to scroll the selection. Since generation is fast, there is no issue with scrolling straight to the bottom and having to wait for the backlog to clear.