va: Get VA display through GstContext
This set of patches enables the fetching of VA display through GstContext, so users can set a custom VADisplay for the pipeline. Otherwise the first opened element will create a DRM VADisplay, as it was before.
Another patch, enables the request of the VASurfaceID from the GstBuffer when mapping with a flag similar to GST_MAP_GL
Also, an example is added, where the application creates a X11 VADisplay and a pipeline using vah264dec and appsink. When appsink receives a new sample, it is rendered using vaPutSurface in a GtkWidget area.
This MR is above !1483 (merged)
Edited by Tim-Philipp Müller