Experimental change to pixmap rendering. Switch from using NSImage to NSBitmapImageRef to store the image. This is closer to the core backing store used to display images. Using this I was able to avoid using lockFocus on the images and instead set the graphics context manually allowing us to draw directly on the backing image. However NSBitmapImageRef is a more simple format and didn't work with the flipped Y axis... so... I had to perform some voodoo to get the flipping to work with the graphics context. Not sure it is 100% right.