Memory leak when porting to GTK 3

A significant amount of the memory used to load an image is not released after it is loaded, whereas it was released before 793ab19e.

You can use any image large enough to see this, for example the one in #60 (closed), or a sample.

Using for example the sample extracted from this image by convert input.jpg -shave 3000x3000 output.jpg, Ristretto occupies 1.4 GiB in memory just before the end of the loading. Then, before 793ab19e, it would release about 750 MiB, after this commit it doesn't release them anymore.