Prep work for serializing display lists across the IPC boundary. Replace
Gfx::Filter's Skia-specific backing with a portable Variant-based
representation, add serialize_filter/deserialize_filter, and store
filter data inline in ApplyEffects/ApplyBackdropFilter command payloads
instead of in DisplayListResourceStorage. FilterResourceId is removed
along with the per-storage filter map.
With the SkImage cache extracted, ImmutableBitmap no longer needs to
hold a SkiaBackendContext, an SkImage, or an SkBitmap. Reduce it to just
the source pixels (a Bitmap or YUVData) and a color space, so the sam
ImmutableBitmap can be consumed by any Skia context without
coordination.
Per-context locking and ensure_sk_image() are no longer needed and go
away with this change; the cache rebuilds an SkImage from the source
data the first time each bitmap is drawn against a context.
CSS filters work similarly to canvas filters, so it makes sense to have
Gfx::Filter that can be used by both libraries in an analogous way
as Gfx::Color.
