do not force all display-targets to be linear
I think !2678 (merged) was a mistake. I accidentally conflated the fact that the winsys displaytarget has to be linear, thinking this also meant the pipe-driver frontbuffer resource had to be linear. It of course does not have to be.
I think @anholt tried to hint at this being a mistake, but I somehow didn't pick up on it.
This is causing issues for D3D12 on WSI, so let's back paddle on this.
I've made Zink a tad less strict here than it used to be, to avoid needless performance regressions.