Blender Git Loki

RNA: Increase memory limit settings to their possible max on platform

This is quite common to have 64GB memory now, and even 128. There is no reason
to add any artificial caps on the cache and undo memory here. We can not protect
against using too much memory in one cases and allow use of full computer
potential in others.

Now 32 bit will use 2GB max (as it used to be), but 64bit will use whatever
number of megabytes fits into integer.

Reviewers: campbellbarton, mont29

Subscribers: sebastian_k

Differential Revision: https://developer.blender.org/D2972