Suboptimal BasisLZ Global Data Layout

The current layout of BasisLZ Global Data prevents eager decoder initialization because its variable-length `imageDescs` array is located in the middle and depends on the prior knowledge of the whole KTX configuration. We should revise it in the future revisions of BasisLZ.

By relocating `imageDescs` to the end of `supercompressionGlobalData` section, loaders would be able to do the following:
1. Upon encountering `sgdByteOffset`, immediately jump (e.g. using a separate thread) to it.
2. Use the contiguous block of data that would start there to initialize BasisLZ endpoints/selectors/tables even before the levels/faces/etc configuration is known.

Review the current BasisLZ Global Data layout and the supercompressionGlobalData section described by the issue. Determine how relocating the variable-length imageDescs array affects the KTX specification and decoder initialization; done means a decided layout revision that enables the proposed contiguous initialization data.

Written by the indexing model from the issue text.