zink: fix emulation of no mipfilter

This approach is taken from the Vulkan spec1, where a robust maxLod of 0.25 is proposed instead of 0.0.

This has the effect of allowing room for both minification and magnification filters, yet still rounding down to the right miplevel in the end.

Fixes: 8d46e35d ("zink: introduce opengl over vulkan")