On the subject of actually supporting software devices, we should probably
implement those by creating a software or reference adapter with appropriate
gl_info and pipe implementations and creating a device based on that.

Just make sure the texture is allocated, so we have something to attach. Note
also that context_apply_attachment_filter_states() runs under the GL lock, so
preloading resources is actually unsafe there.

Similar to surface_prepare_system_memory(). In the long term we'll probably
want to move the whole location management to resource.c and unify buffers and
textures there.

The caller is already responsible for doing this, so it's redundant at best.

Found by Valgrind.