va: enhance image formats negotiation
These patches remove the arbitrary list of image formats to negotiate for raw caps and now gets the available image formats and assumes that the decoder can download to those formats with the same chroma (rtformat) with the native surface format.
The allocator does the inverse operation: from the negotiated raw caps formats, figures out the native surface format.
Also adds the support for libdrm fallback available in gst-build.
Edited by Víctor Manuel Jáquez Leal