st/mesa: remove st_TexImage(), use core Mesa code instead

st/mesa: remove st_TexImage(), use core Mesa code instead

The core Mesa code does the equivalent memory allocation, image mapping,
storing and unmapping.  We just need to call prep_teximage() first to
handle the 'surface_based' stuff.

The other change is to always use the level=0 mipmap image when accessing
individual mipmap level images that are stored in resources/buffers.
Apparently, we were always using malloc'd memory for individual mipmap
images, not resource buffers, before.

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