cogl-color: Add cogl_color_unpremultiply()

For some operations on pre-multiplied colors (say, replace the alpha
value), you need to unpremultiply the color.

This patch provides the counterpart to cogl_color_premultiply().

clutter/cogl/cogl/cogl-color.c

@@ -163,6 +163,17 @@ cogl_color_premultiply (CoglColor *color)
   color->blue = (color->blue * color->alpha + 128) / 255;
 }
 
+void
+cogl_color_unpremultiply (CoglColor *color)
+{
+  if (color->alpha != 0)
+    {
+      color->red = (color->red * 255) / color->alpha;
+      color->green = (color->green * 255) / color->alpha;
+      color->blue = (color->blue * 255) / color->alpha;
+    }
+}
+
 gboolean
 cogl_color_equal (gconstpointer v1, gconstpointer v2)
 {

clutter/cogl/cogl/cogl-color.h

@@ -299,6 +299,19 @@ cogl_color_get_alpha (const CoglColor *color);
 void
 cogl_color_premultiply (CoglColor *color);
 
+/**
+ * cogl_color_unpremultiply:
+ * @color: the color to unpremultiply
+ *
+ * Converts a pre-multiplied color to a non-premultiplied color. For
+ * example, semi-transparent red is (0.5, 0, 0, 0.5) when premultiplied
+ * and (1.0, 0, 0, 0.5) when non-premultiplied.
+ *
+ * Since: 1.4
+ */
+void
+cogl_color_unpremultiply (CoglColor *color);
+
 /**
  * cogl_color_equal:
  * @v1: a #CoglColor

doc/reference/cogl/cogl-sections.txt

@@ -384,6 +384,7 @@ cogl_color_get_alpha_float
 
 <SUBSECTION>
 cogl_color_premultiply
+cogl_color_unpremultiply
 cogl_color_equal
 </SECTION>