clients: Add simple-dmabuf-egl

Add a client that uses EGL/GLESv2 to draw to dmabuf buffers, utilizing
EGLImages and FBOs. The client uses GBM to create the dmabufs buffers.

The simple-dmabuf-egl client is partly based on patch [1] that changes
dmabuf clients to use GBM instead of libdrm code, but has been greatly
simplified since in this case we don't require direct pixel access or
non-RGBA formats.

[1] https://patchwork.freedesktop.org/patch/239796/



Signed-off-by: Alexandros Frantzis <alexandros.frantzis@collabora.com>