Fix bug with cache miss bitmap

• ✅ Dupe bitmaps internally in Bitmap.load method, solving both preloaded and non-preloaded bitmaps being overwritten by ImageManager

This was a "hotfix" for RC7 and so I'm including its changes below too:

• ✅ Hue bug: Fixed by adding a data accessor that returns a "copied" blitted Bitmap when it's accessed. This works fine together with MV's ImageManager, and shouldn't pose issues.
• ✅ Bug during BattleTest mode it may look for battle background image data that is impossible to fetch (there is no current map object available)
• General code cleanup

Download 2.0.0-rc8 here: https://raw.githubusercontent.com/TorD/rmmv-preload-manager/2.0.0-rc8/TDDP_PreloadManager.js