Add do_row_3d for mipmapping 3D textures

Previously 3D textures were mipmapped using multiple passed through
the 2D mipmap generation code.  This had 3 disadvantages.  First, the
extra passes were slow.  Second, this required the allocation of a
temporary buffer to hold intermediate data.  Third, and most
important, the extra passes caused loss of additional bits due to
integer division / bit-shifting.

With this change, our mipmapgen conformance test passes for
non-compressed texture formats.

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