st/dri/sw: Fix pitch calculation in drisw_update_tex_buffer

st/dri/sw: Fix pitch calculation in drisw_update_tex_buffer

swrastGetImage rounds the pitch up to 4 bytes for compatibility reasons
that are explained in drisw_glx.c:bytes_per_line, so drisw_update_tex_buffer
must do the same.

Fixes window skew seen while running firefox over vnc on a 16-bit screen.

NOTE: This is a candidate for the stable branches.

[ajax: fixed typo in comment]

Reviewed-by: Stéphane Marchesin <marcheu@chromium.org>
Signed-off-by: Richard Sandiford <rsandifo@linux.vnet.ibm.com>