cex100: Add a Clutter EGL backend for CE3100/CE4100 SoCs

cex100: Add a Clutter EGL backend for CE3100/CE4100 SoCs

Intel CE3100 and CE4100 SoCs are designed for TVs. They have separate
framebuffers that are blended together by a piece of hardware to make
the final output. The library that allows you to initialize and
configure those planes is called GDL. A EGL GDL winsys can then be
use with those planes as NativeWindowType to select which plane to use.

This patch adds a new ClutterBackendCex100 backend that can be
selected at compile time with the new --with-flavour=cex100 option.