vaapi: [RFC] remove memory:VASurface caps feature

Submitted by Víctor Manuel Jáquez Leal @vjaquez

Link to original bug (#769436)

Description

This is an idea: what if we get rid of the memory:VASurface caps feature?

The purpose of the caps feature is to restrict the type of caps the element can process. But vaapi element can process as input two types of caps: system memory and GstVaapiVideoMemory. As output, three: system memory, GL texture upload and vaapi memory.

Do we need to annotate the vaapi memory?

I think we do not need it, since vaapi surfaces can be mapped onto images (not completely, but almost).

Opinions?