Convert legacy layer JSON before upgrading

The old whimtex.layers clipboard format is no longer supported. Before upgrading, paste it in WhimTex 0.12.5, then save the resulting document as TIFF or export whimtex.document JSON. Use TIFF when the layers contain Drawing pixels.

The current authoring guide and document schema describe whimtex.document, version 1. Its JSON does not download images or store Drawing pixels. A plain HTTP(S) image URL can still be pasted as a Drawing layer; see image paste. Brush and gradient preset formats are unchanged.