surface: add support for GEM buffer exports.

surface: add support for GEM buffer exports.

Add support for GEM buffer exports. This will only work with VA drivers
based off libdrm, e.g. the Intel HD Graphics VA driver. This is needed
to support interop with EGL and the "Desktop" GL specification. Indeed,
the EXT_image_dma_buf_import extension is not going to be supported in
Desktop GL, due to the lack of support for GL_TEXTURE_EXTERNAL_OES targets
there.

This is useful for implementing VA/EGL interop with legacy Mesa stacks,
in Desktop OpenGL context.

https://bugzilla.gnome.org/show_bug.cgi?id=736717