0.94.1 problem with bc4+ dds texture sequences, ati2 headers

[yarrmateys]

there seems to be a problem with handling textures that use these newer compression formats. opening them works fine, but attempting to go to the next or the previous through page up/down will eventually start removing all of them from the browsable list, until you can only browse the older formats like raw or dxt1-5. any of the newer formats will be ignored until manually opened again.

there is also a problem with ATI2 dds texture header. all one needs to do is modify the header so instead of ATI2 it says BC5U and xnview loads it, so perhaps it could include these headers in the supported list, as the support for them is already there.