Approximately three or four times now, the issue of pointers being
in an inconsistent state been an issue in the video backend renderers
with regards to tripping up other developers.

Global (ugh) resources are put into a unique_ptr and will always have a
well-defined state of being - null or not null

We are used to use the texture parameter for all util draw calls,
but AMD seems to have a bug where they use the sampler parameter
of stage 0 if no sampler is bound to the used stage.
So as workaround (and a bit as nicer code), we now use sampler
objects everywhere.

This drops the "feature" to load level 0 from the custom texture
and all other levels from the native one if the size matches.
But in my opinion, when a custom texture only provide one level,
no more should be used at all.