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Fix logic issue with the DXT1a texture format, refs #4549.

Description

Fix logic issue with the DXT1a texture format, refs #4549.

Before the NVTT upgrade, no DXT1a file would have been properly decoded: they would all have been mistaken for a 8bpp greyscale due to a logic issue in our code, which would have triggered a crash. I did not notice the logic issue when performing the upgrade. As a result, decoding those files now fails silently: their alpha bit will be ignored and they will be handled as DXT1.

This patch fixes the logic and allows us to decode DXT1a properly, in accordance with the format specifications. Currently, we do not use this format.

Reviewed By: wraitii
Differential Revision: https://code.wildfiregames.com/D2522