The present flip does not work with the EGLStream backend. Similarly,
the EGLStream backend does not require the buffer to be flushed as
eglSwapBuffers() should take care of this.

Instead of actually checking the backend in use in the present code,
add a flag in the form of a bitfield to the EGL backend to indicate
its features and requirements.

This should not introduce any functional change.

v2: Fix logical test (Adam Jackson <ajax@redhat.com>)

Signed-off-by: Olivier Fourdan <ofourdan@redhat.com>
Reviewed-by: Adam Jackson <ajax@redhat.com>