softpipe: fix glitch in texel lookups on fastpaths

softpipe: fix glitch in texel lookups on fastpaths

Fixes two issues - firstly for mipmap levels with one or more
dimensions smaller than tilesize, the code was sampling off the edge
of the texture (but still within the tile).

Secondly, in the linear_mipmap_linear case, both the default code and
new fastpath were incorrect.  This change fixes the fastpath and adds
a comment to the default path, which still needs to be fixed.
Basically the issue is that the coordinates in the smaller texture
level are/were being computed by just dividing thecoordinates from the
larger texture level by two, as in:

               x0[j] /= 2;
               y0[j] /= 2;
               x1[j] /= 2;
               y1[j] /= 2;

The issues with this are signficant.  Initially x1 is most often equal
to x0+1, but after this, it will likely be equal to x0, so we will not
actually be performing the linear blend within the smaller mipmap.

The fastpath code avoided this (recalculated x1), but was still using
the weighting factors from the larger mipmap level (xw, yw), which
were incorrect.

Change the fastpath code to do two full, independent linear samples of
the two mipmap levels before blending.  The default code needs to do
the same thing.