mesa: Fix glCompressedTexImage when dstRowStride != srcRowStride.

mesa: Fix glCompressedTexImage when dstRowStride != srcRowStride.

Since the MapTextureImage changes on Intel, nwn had corruption in the
scrollbar at the load game menu, and corrupted ground textures in the
starting zone.  Heroes of Newerth's intro screen was also thoroughly
garbled.  A new piglit test "compressedteximage" was created to
regression test this.

The issue was this code now seeing dstRowStride aligned to hardware
requirements instead of a temporary buffer that gets uploaded to
hardware later.  The existing code was just trying to memcpy
srcRowStride * height / bh, while the glCompressedTexSubImage2D()
storage code nearby did the correct walking by blockheight rows at a
time.  Just reuse the subimage upload instead of duplicating that
logic.

v2: Update comment at the top of the function (suggestion by Joel
Forsberg)

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=41451
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org> (v1)