Add condition to default mixins to ensure they're only run once (#240)

As this component is likely to be included by many other components the nested loops inside
of these mixins can have a drastic effect on project build performance.

For example the Sass build step for the FT.com search page app can currently take over 40
seconds but only 1-3 seconds after.

main.scss

@@ -8,10 +8,16 @@
 @import 'src/scss/mixins';
 
 // Set the palette colors programatically
-@include _oColorsSetDefaultPaletteColors();
-@include _oColorsSetPaletteMixes();
-@include _oColorsSetPaletteTones();
-@include _oColorsSetDefaultUsecases();
+// NOTE: Only do this once as this partial will be included by many other components
+// and the multiple nested loops can have a huge effect on compilation performance.
+@if ($_o-colors-is-initialised == false) {
+	@include _oColorsSetDefaultPaletteColors();
+	@include _oColorsSetPaletteMixes();
+	@include _oColorsSetPaletteTones();
+	@include _oColorsSetDefaultUsecases();
+
+	$_o-colors-is-initialised: true !global;
+}
 
 /// Output `o-colors` CSS.
 ///

src/scss/_variables.scss

@@ -3,6 +3,11 @@
 /// @access public
 $o-colors-is-silent: true !default;
 
+/// Ensure default mixins are only called once
+/// @type Bool
+/// @access private
+$_o-colors-is-initialised: false !default;
+
 /// A map to define default palette colours:
 /// A list of arguments for `oColorsSetColor`, except the project name.
 ///