Fix tooltip flicker

[jyaconelli]

closes #116

Issue

When a tooltip is initially rendered in a location on the screen without enough room, it flickers indefinitely.

Solution

proposed by @tailorem in #116 (comment)
The tooltip will only update contentSize when the new size is larger than the previous. This fixes the flickering issue.

[DanUnited]

There is a problem with this solution:
When the tooltip contains only one small string (Text), the width and height of the tooltip container are too big and may not be reduced.