Newly converted .nitro bundles now store their spritesheet as WebP VP8L
instead of PNG, so imports land much smaller without changing a single
pixel. The texture member and spritesheet.meta.image are both labelled
from the actual bytes, never from a caller's assumption.
- encode through sharp with lossless and exact, so colour hidden under
alpha 0 survives; this mirrors ImageSharp's TransparentColorMode.Preserve
- detect PNG/WebP by magic bytes and reject anything the client cannot
render, on create, download and upload paths
- keep the source format when deriving size-32 sheets, scaling composites
and editing metadata, so existing bundles are never silently rewritten
- report fidelity in the studio: the compression panel re-encodes with the
same options the importer uses, so it cannot drift and invent false
warnings, and shows PNG/WebP size estimates
convertSwfToNitro and buildSpritesheet are now async, so the worker, the
main-thread fallback and every import call site await them. PNG stays
supported for existing bundles and icon sidecars are untouched.