CCSprite9Slice will render an image in nine quads, keeping the margins fixed and stretching the center quad to fit the content size.
The effect is that the image’s borders will remain unstretched while the center stretches.
Sets the margin as a normalized percentage of the total image size.
If set to 0.25, 25% of the left, right, top and bottom borders of the image will remain unstretched.
@property (nonatomic, assign) float margin
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Note: Margin must be in the range 0.0 to below 0.5.