Quick etcpak quality test

etcpak is a useful and really fast ETC1 (and some of 2) texture compressor. There is no such thing as a free lunch however, and there are some tradeoffs involved here. Quick example:

Original (kodim03):

crnlib in ETC1 uber mode (8.067 seconds):

RGB: Error: Max:  88, Mean: 2.086, MSE: 9.770, RMSE: 3.126, PSNR: 38.232, SSIM: 0.982703
Y: Error: Max:  34, Mean: 1.304, MSE: 3.750, RMSE: 1.936, PSNR: 42.391, SSIM: 0.982703

etcpak, ETC1 mode only (.006 seconds):

RGB: Error: Max:  80, Mean: 2.492, MSE: 12.757, RMSE: 3.572, PSNR: 37.073, SSIM: 0.980072
Y: Error: Max:  49, Mean: 1.494, MSE: 4.996, RMSE: 2.235, PSNR: 41.144, SSIM: 0.980072

Note I've integrated etcpak directly into my project, and used the BlockData class directly. This thing is *fast*, even without threading!

crnlib has several lower quality settings that are much faster (and still higher quality than etcpak), but nowhere near the speed of etcpak. I've not been focused on pure speed, but on quality and unique features like RDO and intermediate formats like .CRN.

I think the primary value of etcpak is its high performance and relatively compact code size (especially for an ETC2-aware compressor). On many textures/images it'll look perfectly fine. Next up is ETC2Comp, limited to ETC1 mode.