[Impeller] Give filters responsibility over creating passes

[bdero]

No semantic change beyond debug labels and resource creation order. This just moves filter pass creation down the stack from FilterContents::RenderToSnapshot to virtual FilterContents::RenderFilter.

• Filters now create their own render passes and return a snapshot.
• Most filters now resolve their inputs before creating their own textures/render passes.
• The pass textures now have identifiable labels.

This allows filters to:

• Avoid creating textures when not necessary.
• Passthrough inputs, including in cases where the input only needs to be transformed.
• Do arbitrary pass encoding (blit passes, multiple render passes, etc.)

[flutter-dashboard]

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[jonahwilliams]

These changes look reasonable to me but I'm also out of my depth a bit 😅

RSLGTM