[Fixes bug #987606] Refactor the image preview code to avoid loading full-size images.
[Fixes bug #987606] Refactor the image preview code to avoid loading full-size images.

Vastly improves performance for previews of large images, and prevents memory usage spikes.
Also provides a moderate performance improvement for normal size images.

PintaProject/Pinta,Mailaender/Pinta,Fenex/Pinta,PintaProject/Pinta,Mailaender/Pinta,jakeclawson/Pinta,PintaProject/Pinta,Fenex/Pinta,jakeclawson/Pinta