Emulate special handling of oFog (and oPts?) register

[abaire]

Writes to the oFog register take the highest priority component and assign it to x (e.g., writing to oFog.yzw will actually set x to whatever is written to "y")

See https://github.com/abaire/nxdk_pgraph_tests/blob/main/src/tests/fog_tests.cpp
See xemu-project/xemu#981