Implements 2 helper functions for Color type (white() and black())

and uses it in layout_task.rs.

components/gfx/color.rs

@@ -21,3 +21,13 @@ pub fn rgb(r: u8, g: u8, b: u8) -> AzColor {
 pub fn rgba(r: AzFloat, g: AzFloat, b: AzFloat, a: AzFloat) -> AzColor {
     AzColor { r: r, g: g, b: b, a: a }
 }
+
+#[inline]
+pub fn black() -> AzColor {
+    AzColor { r: 0.0, g: 0.0, b: 0.0, a: 0.0 }
+}
+
+#[inline]
+pub fn white() -> AzColor {
+    AzColor { r: 1.0, g: 1.0, b: 1.0, a: 1.0 }
+}