We should invert CMYK data only if color space is JCS_CMYK and either
there is no Adobe marker, or the Adobe transform is 0. Transform 2
indicates YCCK data, which we should not invert.
We have to be careful to always destroy the jpeglib decompression struct
before returning from JPEGLoadingContext::decode. We were doing this in
jpeglib error handlers, but we have a couple of paths that bail from the
decoder via TRY. These paths were neither cleaning up memory nor setting
the image decoder to an error state.
So this patch sets up a scope guard to ensure we free the decompressor
upon exit from the function. And it delegates the responsibility of
setting the decoder state to the caller (of which there is only one),
to ensure all error paths result in an error state.
