zink: non-seamless cube handling

this is the zink-only version of !15416 (closed)

short version: shader keys control rewriting specific cube samplers to 2dArray samplers for non-seamless cube sampling, and now all cube samplerviews are really cube+2dArray samplerviews that can be swapped when necessary

problem: as in #6191 (closed), this breaks miplevels at cube edges because implicit lod is broken (piglit/bin/cubemap pbo):

I tried (top patch) resolving that the same way I did with GL_CLAMP emulation (tex -> txd, txb -> txl), which does smooth out the edges nicely, but this has different issues:

the test in question is supposed to look like this:

any feedback from miplevel experts appreciated