swrast: When asked to map a slice of a 1D array, give back that slice.

swrast: When asked to map a slice of a 1D array, give back that slice.

Until now, we've been treating 1D arrays as a single slice, and each
array slice is actually just a row of the 2D texture.  While swrast
still stores them this way, hardware drivers think that 1D arrays have
actual separate slices not stored as contiguous rows.

Reviewed-by: Brian Paul <brianp@vmware.com>