Draft: tools: Layer group warp

This MR is a crude method of getting layer group warping to work, it does the job but is very particular. All layers in the group are resized to the image size before warping takes place. I'd be grateful for any advice @cmyk.student, or anyone else who can help. I realize this is jumping the gun on whatever NDE warpage plans team GIMP might have, however, even this temporary patch is useful, a feature I can put to daily use.

When painting it's necessary to paint in multiple layers, so that you can have some separation for later adjustments. It's also necessary to adjust shapes to exaggerate or correct forms. The simplest, most playful way to do this is with a warp, across multiple layers.

I spent hours lost in offsets and a maze of stuff I don't understand, in the end I surrendered and went for simple resizing.

Specifically it would be much, much better if:

• layers do not have to be resized by the code for the warp to work
• only those layers which are influenced by the warp are processed
• if the button action is undo-able
• if the active warp, didn't visually pop off, before each layer warp is processed.

layer-group-warp