Draft: core, tools: Layer group warping
A fast method to warp layer groups via a proxy. When a warp is started on a layer group a proxy layer is created. When the warp is committed, the warp is transferred to layers in the folder group. Layers are automatically sized to fit the warped content and layer borders are retained.
Undo is working, but it needs a pro to look at how it should be implemented within the warp tool. There can be error messages about the undo, and the proxy layer may be left over after an undo. It's fast and fun, enhancing the warp tool and enabling a better workflow for painting.
Note: I used a conversion of the layer group buffer to make the proxy, this triggered an assert because the new layer might be assigned a PASS_THROUGH_MODE. Which is not applicable to layers. This was mitigated by adjusting the code in gimplayer.c. It's a lot more responsive to make the proxy this way than doing a copy-paste.