Show pixel info under the mouse when previewing image.

Showing the image tip on the top right of the cursor until there is no room on the right. When no room on the right, then the image tip shows up on the top left of the cursor. We draw two crossing guides to indicate the cursor location. But for an image that is narrow than 20 pixels in width or height, we also show the crosshair cursor since the guides are pretty useless for small images.