textures: ASTC opaque scan from a table of block modes

Choosing BC1 or BC3 for an ASTC texture re-compressed on the GPU scans
every block of it for an alpha endpoint mode when the image is made, on
the GPU thread. Each block's mode was decoded bit by bit
(DecodeBlockInfo): 224 us per new ASTC texture on Mario Kart World's
title screen, which streams textures in by the thousand, and ~10% of the
time in its frames over 100 ms.

DecodeBlockInfo reads nothing but a block's 11 mode bits, so its result
for all 2048 of them is worked out once; the scan takes the partition
count and endpoint modes straight from the block's first word.

Same answer as before for all 2^29 values of the bits the scan reads
(random data in the rest, 538M blocks). A 4 MiB opaque texture scans in
1.6 ms instead of 12 ms; the title screen's new ASTC textures in 37 us
each instead of 224.

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