Use WebKit translate with Accelerated Compositing to rearange tiles

WebKit support GPU acceleration when using WebKit transform with
3D transforms.

This patch use WebKit Transform when possible in order to improve
the performance when moving the tiles.
There is no need to positionTile() after prepareTiles(), it is already done
Simplify the handling of blank image in assignTileImage()

The branch if(!useBlankImage) is always taken since useBlankImage
was assigned just above.

Move useBlankImage, tileImgId and src to improve the span of those
variables.
Add a Copyright :)
No need to force the size when the scale is 1

If the final size is equal to the natural size, forcing the size
with CSS is useless.

Since the style is not shared here, that was just wasting memory
and adding complexity.
Generalize the change of position through CSS Transforms

To avoid re-rendering the ThumbnailControl all the time, the
navigation rect is moved through CSS transform.
Ensure no layer are left behind when reseting.

Loading images could stay in the well and polute the tree.
This patch ensure everything is cleared before recreating the
tiles.
Merge pull request #4 from Ikipou/master

Some perf improvement