zink: enforce viewport depth clamping
VUID-VkViewport-minDepth-01234 specifies that depth must be in the range [0.0, 1.0], so the viewport must always be clamped to this range
this affects texture clears using u_blitter, as this expects to be able to use the GL range of [-1.0, 1.0], so pass the depth value as though it's been de-converted back to a GL z coordinate to account for viewport transform
cc: mesa-stable
fixes #6757 (closed)