Only allow one modifier per DMA-BUF, split attributes struct in render/
The DMA-BUF attributes struct is starting to be used in various parts of wlroots, some of which have nothing to do with the linux-dmabuf protocol. This extracts the common struct into wlr/render/dmabuf.h
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This also removes the ability to have per-plane modifiers.
Fixes #908 (closed)