It's to allow image loader to check for sane limits for image sizes.
Idea came from this patch from sfan5 for Minetest: dbd39120e7
Thought solution is a bit different.
Image loader checks not yet added (will come soon).
Also note that limit is to s32. While u32 might work mostly it will run into some troubles with color converter for now (which maybe could be changes). Also 2GB ought to be enough for anybody, right?
git-svn-id: svn://svn.code.sf.net/p/irrlicht/code/trunk@6386 dfc29bdd-3216-0410-991c-e03cc46cb475