Adds a sunxi-cedrus-drv-video libVA backend

This VA backend uses v4l2's Frame API proposal to interface with the
"sunxi-cedrus" video driver on Allwinner SoC. Only a few parts of the
code are really dependent on sunxi-cedrus and this VA backend could be
reused for other v4l drivers using the Frame API.

README.md

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+Sunxi Cedrus VA backend
+=======================
+
+This libVA video driver is designed to work with the v4l2 "sunxi-cedrus" kernel
+driver. However, the only sunxi-cedrus specific part is the format conversion
+from tiled to planar, otherwise it would be generic enough to be used with other
+v4l2 drivers using the "Frame API".
+
+You can try this driver with VLC but don't forget to tell libVA to use this
+backend:
+
+	export LIBVA_DRIVER_NAME=sunxi_cedrus