Maintain hierarchical structure of sprite groups instead of flattening.
Allows opacity and blend mode to be applied correctly to groups.
Sets the foundation for future features like mask layers.
Note:
Requires full image rendering and impacts performance in some scenarios.
Avoids complex code changes for minor performance gains.
Co-authored-by: Guilherme Marcondes <guilherme.marcondes@tecnico.ulisboa.pt>
Fix regression introduced in 24846eae10c7340a4e7f103b15494ea8338700a9
using the new RenderPlan structure. We have to include empty/nullptr
cels in the plan to paint the preview image in the selected
layer/frame (the preview image is the one used for the brush preview,
and for the transformation preview when we paste the clipboard content
on the canvas).
An empty cel must be counted as a layer for the z-index ordering, so
the z-index refers to number of layers to move back/front, but number
of non-empty cels in the specific frame.
A this fix, a new issue appears: #3820
* Now a Cel has a z-index property to change the order of layers per frame
* A new doc::RenderPlan class can calculate the order of cels to be rendered
* z-index is saved as a int16_t in the .aseprite files
* This new field can be set/get from Lua with Cel.zIndex
