gl-renderer, pixman: disconnect the client on unhandled buffer type.

Introduce a helper function to disconnect the client on unhandled
buffer types, and use it in the gl and pixman renderers. The function
is modeled after linux_dmabuf_buffer_send_server_error.

Also print the egl error state in the gl renderer, in case the
unrecognized buffer error happens when querying an egl buffer.

wayland/weston#148
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