Add wayland-server support to nullws

Adds support for running a wayland compositor off of nullws.
This allows for platforms that have trouble bringing up fbdev and
hwcomposer to run a wayland compositor using null EGL platform for the
compositor.

tested using nexus5, qml-compositor and hellogl_es2 client.
Merge pull request #233 from heeen/null-compositor

Add wayland-server support to nullws
Cleanup the buffers from the list once destroyed

Since we dispatch the wayland queue in destroyBuffer() a wl_buffer.release
event may come while we're still mid-through deleting all the buffers.
When getting a release event we go through the list of the buffers, and so
we risk of accessing a deleted buffer object. To fix this remove the deleted
buffers from the list as soon as possible.