mesa: Make formats.c "datatype" values match glGetTexLevelParameter return.

mesa: Make formats.c "datatype" values match glGetTexLevelParameter return.

The formats.c code's "datatype" value is "what does this value mean",
i.e. unorm or snorm or float, and is the return value from the
GL_TEXTURE_RED_TYPE class of queries.  The depth formats were marked
as GL_UNSIGNED_INT, which is what we use for integer, and not what we
should be returning from the glGetTexLevelParameter.

In texstore, we were inappropriately using it as an argument to
_mesa_unpack_depth_span() that was expecting a value like
GL_UNSIGNED_INT or GL_UNSIGNED_SHORT.  Just hardcode
_mesa_unpack_depth_span()'s arguments for now, though it looks like
the consumers of that interface would be happier with using
MESA_FORMAT.

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