bug#33275: 27.0.50; Image cache pruning

[Eli Zaretskii]

> Date: Mon, 05 Nov 2018 20:14:27 +0200
> From: Eli Zaretskii <eliz@gnu.org>
> Cc: 33275@debbugs.gnu.org
> 
> I don't see why we should tweak our cache management for the benefit
> of exceptional use case.  Last time we did it (with fonts) we are
> still leaking the wounds.
        ^^^^^^^
I meant "licking", of course ;-)

> I think it should be enough to catch excess memory usage and signal an
> error.  I hope this is possible.

I guess we lost the ability to do that automatically when we stopped
using vm-limit.c on modern platforms?

We still could use the memory limit calculated by get_lim_data in
strategic places, like cache_image, to catch situations where the
cache size exceeds some fraction of what ret_lim_data returns.  Would
that work?