Fix face bytelength calculation for PVRTC1

[lexaknyazev]

KTX-Software/lib/texture2.c

Lines 2018 to 2019 in 4e0a2ce

	faceSize = blockCount.x * blockCount.y
	* prtctd->_formatSize.blockSizeInBits / 8;

From the PVRTC1 spec:

For PVRTC 4BPP formats the imageSize is calculated as:
(max(width, 8) * max(height, 8) * 4 + 7) / 8
For PVRTC 2BPP formats the imageSize is calculated as:
(max(width, 16) * max(height, 8) * 2 + 7) / 8

[MarkCallow]

Are width & height in pixels? If so, this seems a really strange calculation as it easily results in a value that is not a multiple of the block size.

[lexaknyazev]

This calculation limits the minimum payload size because any PVRTC1 texture must comprise at least two words of PVRTC1 data in each dimension.

[lexaknyazev]

Additionally, the dimensions of PVRTC1 data must always be powers-of-two, so the result is always a multiple of 8.

[MarkCallow]

Ah! yes. I was forgetting about the powers-of-two requirement.

[MarkCallow]

I guess I shouldn't have put "issue" between Fixes and the number since the auto close failed.