Basic UI Functionality and small Renderer Optimization

[ulikk]

Just to keep track

• Pause Button still doesn't work
• "Clear Painting" Button would be nice
• ImageSource should be refactored that it also holds the "background" image
• One Image in ImageSource directly caches the Image Contents in an Array, so that the renderer can directly access image data from it and doesn't resort to getImageData() multiple times when drawing a brush. The painted image is only once transferred to the screen via the painter

[juliankrispel]

background image? Which one is that? Is it the destination image?