Add Sprite.pixelSnapping.

This fixes blurriness in images (especially text) on WebGL and Stage3D
when positioned on subpixels. The canvas renderer has always snapped.

This is on by default. It's way better to have to opt-in to
subpixel-smooth animation than have to opt-out of images looking like a
fuzzy mess.
Documentation tweaks.
Only dirty the text layout if things really changed.