By default, image tiles are at least as high resolution as the screen. For a retina screen this may not be
necessary, and may increase the likelihood of an OutOfMemoryError. This method sets a DPI at which higher
resolution tiles should be loaded. Using a lower number will on average use less memory but result in a lower
quality image. 160-240dpi will usually be enough. This should be called before setting the image source,
because it affects which tiles get loaded. When using an untiled source image this method has no effect.