chore: Sync with upstream/official Snap

[brunvonlope]

Ported from: https://gitlab.gnome.org/GNOME/gimp/-/blob/gimp-3-0/build/linux/snap/snapcraft.yaml?ref_type=heads

See the context: #447 (comment)

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The structure is widely different but it results in funciontally the same (or even more featured) Snap as per @frenchwr and mine tests. The only difference is that we don't self build libheif (or any dep other than babl and gegl) regardless of the bug the custom build aimed to fix.

[jnsgruk]

One final question: I'm surprised by the removal of the ffmpeg integration? This is sort of a "free win" for use cases that need ffmpeg - is there a reason to drop that?

[brunvonlope]

This is sort of a "free win" for use cases that need ffmpeg - is there a reason to drop that?

@jnsgruk Nothing on GIMP snap links to libraries from ffmpeg.

From https://gitlab.gnome.org/GNOME/gimp/-/commit/2e5ac18d787699cb9afd57d97b7f682782e24984

libheif from GNOME SDK Snap can't directly use codecs from ffmpeg due
to the use of plugin system (plugins on LIBHEIF_PLUGIN_PATH) at build time.

Also, even if it could (by custom building, which I don't plan to do),
codecs like libde265/x265 are always present in GNOME runtime Snap
(unlike GNOME runtime flatpak) so there is no way of not installing them.