* Blend group layers on `DrawingAlgos.blend_layers()`
* Support group layer blending on the canvas
* Allow editing of group layer properties
* Fix issues with group layer blending in canvas, and unite common code
* Group layers can now be used as clipping masks
* Make move tool preview work on child layers
* Change OffsetImage's `blend_layers()` to support group layer blending
* Support group layer blending in the canvas preview
* Fix layer blending mode, clipping mask opacity and cel opacity not being updated automatically if the layer/cel changed is not selected
* Add a pass through blending mode to layer groups
Fingers crossed that no bugs were introduced
* Fix issue with layers that belong to pass through groups not updating their textures on the canvas automatically on undo
You currently cannot move multiple cels on different layers, they still act as swapping. Eventually I'd also like to implement moving cels between different layers, but they would also have to be on the same frame. I don't think moving cels between different layers and frames at the same time is possible.
A slightly breaking change for a minor update, but one that needed to be done. The bucket tool's "similarity" (now renamed to "tolerance") used to work the opposite way from all other software, the maximum value meant exact color match and 0 meant no color match. Now it works the inverse way to make it be consistent with other software, and the range is now 0-255 instead of 0-100. 0 means exact color match, 255 means no color match. And tolerance also now works for the "similar area" mode as well.
1) `Palettes`'s `reindex_colors_on_width_increase()` was not changing the `index` of `PaletteColors`, thus resulting in the opened palette itself and the saved file having different indices, which could've caused conflicts with multiple colors sharing the same index, if the user re-arranged the palette after resizing it.
2) If the width was increased but the height increased, the positions of the colors remained the same, which resulted in the colors being in the columns that were removed, to be removed themselves. Now, the colors are taking advantage of the empty space, in order to remove as less as colors as possible. No colors will be removed if the width times the height is equal to or greater than it was before.
Thanks to https://godotshaders.com/shader/color-manipulator/
The shader has more options than just brightness and contrast though, but I didn't know how else to name the effect. "Adjust Brightness/Contrast" makes it immediately obvious as to what the effect is about.
