Layershot crashes when the psd size becomes too large

[vpdn]

Currently, because of the way the psd lib works, all psd layers are kept in memory and only written to disk after all layers have been processed. This causes files with large number of layers to fill up the entire space, leading to a low memory situation.

We need to either a check for low memory situations in which case we should abort the psd creation or alter the psd generator to work in a stream based manner (NSOutputStream).