FillSpread example is required

[hermet]

For a showcase, FillSpread example has been missed. We need add in thorvg/examples/FillRules

/**
 * @brief Enumeration specifying how to fill the area outside the gradient bounds.
 */
enum class FillSpread
{
    Pad = 0, ///< The remaining area is filled with the closest stop color.
    Reflect, ///< The gradient pattern is reflected outside the gradient area until the expected region is filled.
    Repeat   ///< The gradient pattern is repeated continuously beyond the gradient area until the expected region is filled.
};