Add support for most of the border-image properties, pass to WR.

This adds support for:
 * border-image (images, not gradients yet)
 * border-image-source
 * border-image-slice
 * border-image-repeat (stretch + repeat only for now)

Remaining work:
 * Connect border-image-outset (WR supports this).
 * border-image-width

components/gfx/display_list/mod.rs

@@ -891,15 +891,9 @@ pub struct GradientDisplayItem {
     pub stops: Vec<GradientStop>,
 }
 
-/// Paints a border.
+/// A normal border, supporting CSS border styles.
 #[derive(Clone, HeapSizeOf, Deserialize, Serialize)]
-pub struct BorderDisplayItem {
-    /// Fields common to all display items.
-    pub base: BaseDisplayItem,
-
-    /// Border widths.
-    pub border_widths: SideOffsets2D<Au>,
-
+pub struct NormalBorder {
     /// Border colors.
     pub color: SideOffsets2D<ColorF>,
 
@@ -912,6 +906,50 @@ pub struct BorderDisplayItem {
     pub radius: BorderRadii<Au>,
 }
 
+/// A border that is made of image segments.
+#[derive(Clone, HeapSizeOf, Deserialize, Serialize)]
+pub struct ImageBorder {
+    /// The image this border uses, border-image-source.
+    pub image: WebRenderImageInfo,
+
+    /// How to slice the image, as per border-image-slice.
+    pub slice: SideOffsets2D<u32>,
+
+    /// Outsets for the border, as per border-image-outset.
+    pub outset: SideOffsets2D<f32>,
+
+    /// If fill is true, draw the center patch of the image.
+    pub fill: bool,
+
+    /// How to repeat or stretch horizontal edges (border-image-repeat).
+    #[ignore_heap_size_of = "WebRender traits type, and tiny"]
+    pub repeat_horizontal: webrender_traits::RepeatMode,
+
+    /// How to repeat or stretch vertical edges (border-image-repeat).
+    #[ignore_heap_size_of = "WebRender traits type, and tiny"]
+    pub repeat_vertical: webrender_traits::RepeatMode,
+}
+
+/// Specifies the type of border
+#[derive(Clone, HeapSizeOf, Deserialize, Serialize)]
+pub enum BorderDetails {
+    Normal(NormalBorder),
+    Image(ImageBorder),
+}
+
+/// Paints a border.
+#[derive(Clone, HeapSizeOf, Deserialize, Serialize)]
+pub struct BorderDisplayItem {
+    /// Fields common to all display items.
+    pub base: BaseDisplayItem,
+
+    /// Border widths.
+    pub border_widths: SideOffsets2D<Au>,
+
+    /// Details for specific border type
+    pub details: BorderDetails,
+}
+
 /// Information about the border radii.
 ///
 /// TODO(pcwalton): Elliptical radii.