Parses the input string for a gradient spread value, which indicates what happens if the gradient starts or ends inside the bounds of the target rectangle.
The allowable values are pad | reflect | repeat
If the string is empty, the default value (DefValue) is returned.
Possible values are: 'pad', which says to use the terminal colors of the gradient to fill the remainder of the target region, 'reflect', which says to reflect the gradient pattern start-to-end, end-to-start, start-to-end, etc. continuously until the target rectangle is filled, and repeat, which says to repeat the gradient pattern start-to-end, start-to-end, start- to-end, etc. continuously until the target region is filled.
If the attribute is not specified, the effect is as if a value of 'pad' were specified.