Fix image memory use (#2045)
Fix image memory use (#2045)

* Add a method for freeing the image file, free each derivative after it has been created

* Save the scaled derivative on disk before freeing it, store its path

tomzx/blog.tomrochette.com,tomzx/blog.tomrochette.com,Vivalldi/grav,getgrav/grav,tomzx/blog.tomrochette.com,stell/grav,getgrav/grav,Vivalldi/grav,tomzx/blog.tomrochette.com