swrast: Make a teximage's stored RowStride be in terms of bytes per row.

swrast: Make a teximage's stored RowStride be in terms of bytes per row.

For hardware drivers with pitch alignment requirements, a
non-power-of-two-sized texture format won't end up being an integer number
of pixels per row.  Also, avoids having to change our units between
MapTextureImage's rowStride and swrast's RowStride.

This doesn't fully convert the compressed texel fetch path, but does make
sure we don't drop any bits (not that we'd expect to).

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Brian Paul <brianp@vmware.com>