mesa: get rid of imageOffsets arrays in texstore code

mesa: get rid of imageOffsets arrays in texstore code

These were used to find the start of a 3D image slice (or 2D array texture
slice) given a base address.  Instead, use a simple array of address of
image slices instead.

This is a step toward getting rid of the gl_texture_image::ImageOffsets
field.

Reviewed-by: Eric Anholt <eric@anholt.net>