V4l2: add to handle dmabuf capsfeature
It includes two patches on the v4l2object and v4l2src. I copied each commit messages as below for description.
The v4l2 elements were able to export dmabuf using io-mode property or do it automatically but it has been negotiated with a normal type of caps without dmabuf features. This can cause a problem when connecting with waylandsink. e.g. If the wayland server supports specific video format of dmabuf memory type though not supports the same video format of system memory type, it will fail to negotiate even if the v4l2 device manages to export dmabuf. It adds dmabuf caps features on the probe_caps if the v4l2 device can handle dmabuf exporting. This prevents caps negotiation failures of dmabuf type memory and helps to negotiate with the correct one.
The dmabuf caps features have been lost while sorting candidate caps to get nearest preference caps. It separates caps into GstStructure, merged them back into cap but the caps features was missing in these steps. This change makes to preserve the dmabuf caps features after sorting the GstStructures of the caps.