swrast: Always use MapTextureImage for mapping textures for swrast.

swrast: Always use MapTextureImage for mapping textures for swrast.

Now that everything goes through ImageSlices[], we can rely on the
driver's existing texture mapping function.

A big block of code goes away on Radeon that looks like it was to deal with
the validate that happened at SpanRenderStart, which no longer occurs since we
don't need validation for the MapTextureImage hook.

v2: Rewrite comment about ImageSlices, fix duplicated swImages, touch up
    unmap loop.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org> (v1)
Reviewed-by: Brian Paul <brianp@vmware.com>