GetWICCodec equivalent for extensions

[walbourn]

It would be useful to be able to get the correct WIC GUID to use for SaveToWIC* based on the common extension name: "jpg", "jpeg", etc.

[walbourn]

Could also make SaveToWICFile take a nullptr for the guidContainerFormat parameter and use the extension in szFile.

[thomasmfields]

This would be really handy actually.