"memory full" during redisplay

[Eli Zaretskii]

While investigating bug #19322, I found out that memory-allocation
functions called in a few places during redisplay will signal a
memory-full error when there's not enough free memory to satisfy a
request.

However, signaling an error in the middle of redisplay is not a good
idea, because Emacs will immediately re-enter redisplay, again signal
an error, etc.  This causes Emacs to lock hard, and the only thing the
user can do is forcibly terminate the process.

So I think we should modify memory_full to handle this differently
during redisplay.  Do we already have some machinery for this, or do
we need to invent something new?