mesa: fix SwapBytes handling in numerous places

mesa: fix SwapBytes handling in numerous places

In a number of places the SwapBytes handling didn't handle cases with
GL_(UN)PACK_ALIGNMENT set and 7 byte width cases aligned to 8 bytes.

This adds a common routine to swap bytes a 2D image and uses this
code in:

texture storage
texture get
readpixels
swrast drawpixels.

[airlied: updated with Brian's nitpicks].

Cc: "11.0" <mesa-stable@lists.freedesktop.org>
Reviewed-by: Brian Paul <brianp@vmware.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>