Now that we have identified the device types associated with each hwdec, we
can take advantage of this to ensure that get the right hwdec is chosen when
decoding frames and there are multiple choices available with different device
types.
This is also plumbed into the filter initialisation logic, although in practice
the decision on the hwdec has already been made at this point - still it's
easier than retaining the ambiguous look up logic and worrying that it might
lead to an inconsistent result.
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