main: Changed _mesa_alloc_texture_storage to _mesa_AllocTextureStorage_sw.

main: Changed _mesa_alloc_texture_storage to _mesa_AllocTextureStorage_sw.

In order to implement ARB_DIRECT_STATE_ACCESS, many GL API functions must now
rely on a backend that both traditional and DSA functions can use. For
instance, _mesa_TexStorage2D and _mesa_TextureStorage2D both call a backend
function _mesa_texture_storage that takes a context and a texture object as
arguments.  The backend is named _mesa_texture_storage so that Meta can call
it and avoid looking up the context and the texture object.  However, backend
names often look very close to the names of software fallbacks (ie.
_mesa_alloc_texture_storage).  For this reason, software fallbacks have been
renamed for clarity to have the form _mesa_[Driver function name]_sw.

Reviewed-by: Anuj Phogat <anuj.phogat@gmail.com>