STRATUM_OFFSET is 3bit while 2bit is enough

[Nielsbishere]

https://github.com/cschied/q2vkpt/blob/master/src/refresh/vkpt/shader/asvgf.glsl#L24 defines it as 3, but for 0-2 (0-GRD_DWN-1) only 2 bits are needed. This normally wouldn't matter, but with bigger resolutions (e.g. 8kx8k), this would be hit. This is because for the 1D location in previous frame it uses a 32 bit int and packs 7 bits for 1 (dirty), 2x3 (x and y GRD_DWN), leaving only 25 bits for texture location. This would not be hit by regular 8k though, as it just about fits, but I'd not recommend it for future proofing.