In gl_texture_image, replace ImageStride with an ImageOffsets array.

In gl_texture_image, replace ImageStride with an ImageOffsets array.
Some hardware lays out 3D mipmaps in a manner that can't be expressed
with a simple image stride.
The ImageOffsets array is allocated and initialized to typical defaults
in the _mesa_init_teximage_fields() function.  If needed, a driver will
then have to replace these offsets.
TexStore and TexelFetch routines updated to use offsets array.