simple-hdf-server.py: Remove the 'all frames' mode, it will be incompatible with 2D grid subgroup output and was not a success anyway
lightsheet slice shader: Fix slice linear combination being applied in reverse
Change lightsheet group texture format from thin stripe to 2D grid

This will help with compatibility with platforms with MAX_TEXTURE_SIZE
as low as 2048 (e.g. many Macs).

I just hardcoded the row length to be 8 as that seems to be perfect
fit with our current resources and slice size 128x128.