Update scripting reference (SuperTux/supertux@02fd5c7)

Author: SuperTux-bot

ScriptingGradient.md

@@ -25,12 +25,12 @@ Method | Explanation
 -------|-------
 `void set_direction(string direction)` | Sets the direction of the gradient. <br /><br /> `direction` - Can be "horizontal", "vertical", "horizontal_sector" or "vertical_sector". 
 `string get_direction()` | Returns the direction of the gradient. Possible values are "horizontal", "vertical", "horizontal_sector" or "vertical_sector". 
-`void set_color1(float red, float green, float blue)` | Set top gradient color. 
-`void set_color2(float red, float green, float blue)` | Set bottom gradient color. 
-`void set_colors(float red1, float green1, float blue1, float red2, float green2, float blue2)` | Set both gradient colors. 
-`void fade_color1(float red, float green, float blue, float time)` | Fade the top gradient color to a specified new color in `time` seconds. 
-`void fade_color2(float red, float green, float blue, float time)` | Fade the bottom gradient color to a specified new color in `time` seconds. 
-`void fade_colors(float red1, float green1, float blue1, float red2, float green2, float blue2, float time)` | Fade both gradient colors to specified new colors in `time` seconds. 
+`void set_color1(float red, float green, float blue, float alpha = 1.f)` | Set top gradient color. 
+`void set_color2(float red, float green, float blue, float alpha = 1.f)` | Set bottom gradient color. 
+`void set_colors(float red1, float green1, float blue1, float red2, float green2, float blue2, float alpha1 = 1.f, float alpha2 = 1.f)` | Set both gradient colors. 
+`void fade_color1(float red, float green, float blue, float time, float alpha = 1.f)` | Fade the top gradient color to a specified new color in `time` seconds. 
+`void fade_color2(float red, float green, float blue, float time, float alpha = 1.f)` | Fade the bottom gradient color to a specified new color in `time` seconds. 
+`void fade_colors(float red1, float green1, float blue1, float red2, float green2, float blue2, float time, float alpha1 = 1.f, float alpha2 = 1.f)` | Fade both gradient colors to specified new colors in `time` seconds. 
 `void swap_colors()` | Swap top and bottom gradient colors.