Call eglCreateImageKHR with no EGL context

For a EGL_WAYLAND_BUFFER_WL target eglCreateImageKHR should be called
with EGL_NO_CONTEXT because no client API context is required and
providing an EGL context results in a EGL_BAD_CONTEXT for some EGL
implementations (i.e. Vivante on Freescale i.MX6).

Change-Id: Ie84c88fd20a470c0c052222ac9365625020f5bb9
Reviewed-by: Jørgen Lind <jorgen.lind@digia.com>