i965: Fix glCompressedTexSubImage2D offsets for ETC textures.

This patch fixes intel_miptree_unmap_etc() (which decompresses ETC
textures to linear) to pay attention to map->x and map->y when writing
to the destination image.  Previously these values were ignored,
causing the xoffset and yoffset parameters passed to
glCompressedTexSubImage2D() to be ignored.

Reviewed-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
WIP: allow varying structs