layout: support tiled gradients

Use background-size, background-position properties to render
CSS gradients.

Some cleanup in display_list_builder.rs related to gradient
calculations.

Adds two wpt tests for tiled gradients.

Note: For now even gradients with background-repeat: no-repeat
are repeated. Sometimes the gradient is not repeated everywhere.

Enable vars-background-shorthand-001.html CSS test.

components/gfx/display_list/mod.rs

@@ -720,6 +720,15 @@ pub struct GradientDisplayItem {
 
     /// Contains all gradient data. Included start, end point and color stops.
     pub gradient: Gradient,
+
+    /// The size of a single gradient tile.
+    ///
+    /// The gradient may fill an entire element background
+    /// but it can be composed from many smaller copys of
+    /// the same gradient.
+    ///
+    /// Without tiles, the tile will be the same size as the background.
+    pub tile: Size2D<Au>,
 }
 
 /// Paints a radial gradient.
@@ -745,6 +754,15 @@ pub struct RadialGradientDisplayItem {
 
     /// Contains all gradient data.
     pub gradient: RadialGradient,
+
+    /// The size of a single gradient tile.
+    ///
+    /// The gradient may fill an entire element background
+    /// but it can be composed from many smaller copys of
+    /// the same gradient.
+    ///
+    /// Without tiles, the tile will be the same size as the background.
+    pub tile: Size2D<Au>,
 }
 
 /// A normal border, supporting CSS border styles.