miptree, add bc3 test, larger images, and intermediate image copy steps

This brings in 3 features as well a KTX image loader.

In order to test compressed textures, I ported the piglit ktx loader over, and added a BC3 texture and changed the miptree test to support it.

This is built on top of two other enhancements, one to test a 16k width or height image, and one to add an extra copy image step.