I've tried fbo_firecube from mesa-demos on Software Rasterizer, llvmpipe, softpipe and Intel HD Graphics 4600, and setting GL_FOG_HINT to GL_NICEST (toggled by pressing 'n') doesn't change anything. OTOH, on nvidia binary driver for an NVidia card I get nice per-pixel fog in this demo. I believe that even though Mesa might refuse to support per-pixel fog in software renderers, at least drivers for the hardware capable of it must support it. This includes i965 and i915, and a bunch of others.
GL_FOG_HINT is just a hint and nothing more so doing nothing with it is perfectly valid. Perhaps we could do better (I honestly don't know). However, since there are basically no modern applications using fixed-function fog, there is very little reason for is to care about making it better at this point. Patches to improve fog handling would probably be welcome of someone cared to write them but I wouldn't expect any one of the main devs to care for the reasons already mentioned.
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