As the first step towards handling scenarios where the are multiple hwdecs for
a given image format but backed by different AVHWDeviceTypes, let us annotate
the hwdecs with their corresponding device types.
From this, we can also see how all the existing hwdecs which match the same
image format also match the same device type.
We will need the full ra_ctx to be able to look up all the state
required to initialise an ffmpeg vulkan hwcontext, so pass let's
pass the ra_ctx instead of just the ra.
Philip Langdale