plugins: check if negotiate dmabuf with downstream

plugins: check if negotiate dmabuf with downstream

In order to enable, in the future, dmabuf-based buffers, the vaapi base
plugin needs to check if downstream can import dmabuf buffers.

This patch checks if downstream can handle dmabuf, by introspecting the
shared GL context. If the GL context is EGL/GLES2 and have the extension
EGL_EXT_image_dma_buf_import, then dmabuf can be negotiated.

Original-patch-by: Julien Isorce <j.isorce@samsung.com>