OpenEXR 2.2 support, esp. for the new DWAA/DWAB compression modes.

When compiled against OpenEXR 2.2, the "dwaa" and "dwab" compression are available. Also respects the "openexr:dwaCompressionLevel" float attribute to set the compression level.

For those not up on it, OpenEXR 2.2 was recently released, and it contains two new compression methods contributed by DreamWorks Animation, which are extremely good at compressing HDR images. It's lossy, but has very high visual fidelity with impressive compression ratios.