Fixes for OpenJpeg/Jpeg2000

First issue is rather simple, just missing search path for multiarch systems.

Second one is a bit more tricky, and it's related on the pre-multiplied (associated) alpha. As far as i concerned Jpeg2000 is expected to have straight alpha, so we need to premultiply (associate it) in order to deliver proper import result. Now it happens in the same exact way as what's happening in PNG importer.

Some code could be de-duplicated perhaps, but not really sure what's the best way to do this.