- Invert bookmark icon logic: show bookmark only when no cover image
(as visual decoration), hide when cover is displayed to show more art
- Replace GfxRenderer::storeBwBuffer usage with HomeActivity's own
buffer management (storeCoverBuffer/restoreCoverBuffer/freeCoverBuffer)
- Remove copyStoredBwBuffer() and freeStoredBwBuffer() from GfxRenderer
as they enabled misuse of the grayscale buffer storage mechanism
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