softpipe: fix misleading TGSI_QUAD_SIZE usage

softpipe: fix misleading TGSI_QUAD_SIZE usage

All these img filter loops iterate through NUM_CHANNELS, not QUAD_SIZE.
In practice both are of course the same unchangeable value (4), but it
makes the code look a bit confusing. Moreover, some of the functions were
actually given an array of 4 values according to the declaration, yet the
code was addressing values 0/4/8/12 out of it, so fix this by just saying
it's a pointer to floats like the other functions.

While here, also add comment about not quite correct filtering.

There's no actual code difference.

Reviewed-by: Jose Fonseca <jfonseca@vmware.com>