WGSL textureSample tests

Add all the textureSample tests. cube and cube-array tests with
derivatives are currently skipped or filtered out as the software
rasterizer can't handle this case correctly or at least doesn't
match too many GPUs. Rather than increase the tolerances I'm
hoping to find something I can measure, like the mapping between
derivative and mix-weight, that will give us some way to test
per GPU.

There's a big change to the soft rasterizer to compute derivatives
by setting up 2x2 pixels and computing the derivatives by the
differences between the directions.