Inappropriate block size for ScanConverterNode

[xiaoyun2]

I try to change the block size of ScanConverterNode from (1,256) to (16,8) which reach a great performance improvements in my side GTX 1080, 1.2ms to 0.3ms and TX2 25.50ms to 3.16ms.
So this may be common that current block size cause performance drop.

[goeblr]

Oh wow!
Yes, I didn't fully tune all the block-sizes.
Is this with 2D imaging?

If you are doing any more tweaks, I'm happy to incorporate a pull-request :)