Ui

[julianko]

this PR improves a couple of things about the UI:

• If you move your mouse one pixel between mouse down and mouse up when clicking on an animation cell, it counts as dragging the cell. This happens very often on accident (at least to me). I've added a threshold of 10 pixels that you need to move at least to make it count as dragging.
• Change the mouse wheel zoom behaviour to zoom towards the cursor instead of the origin of the image.
• Change the way the checkerboard background is drawn. The new method only requires a tiny 2x2 BufferedImage that is used with appropriate tiling to draw the entire checkerboard in one simple fill command. This is requires less code and memory, and should also be more efficient because it's cache friendlier.