Fix canvas image tests when using webrender.

When webrender is enabled, image decoding doesn't pre-multiply by
alpha, but the canvas code expects the image data to be
pre-multiplied form.

components/canvas_traits/lib.rs

@@ -552,3 +552,21 @@ pub fn byte_swap(data: &mut [u8]) {
         i += 4;
     }
 }
+
+pub fn byte_swap_and_premultiply(data: &mut [u8]) {
+    let length = data.len();
+
+    let mut i = 0;
+    while i < length {
+        let r = data[i + 2];
+        let g = data[i + 1];
+        let b = data[i + 0];
+        let a = data[i + 3];
+
+        data[i + 0] = ((r as u32) * (a as u32) / 255) as u8;
+        data[i + 1] = ((g as u32) * (a as u32) / 255) as u8;
+        data[i + 2] = ((b as u32) * (a as u32) / 255) as u8;
+
+        i += 4;
+    }
+}

components/script/dom/canvasrenderingcontext2d.rs

@@ -5,7 +5,7 @@
 use canvas_traits::{Canvas2dMsg, CanvasCommonMsg, CanvasMsg};
 use canvas_traits::{CompositionOrBlending, FillOrStrokeStyle, FillRule};
 use canvas_traits::{LineCapStyle, LineJoinStyle, LinearGradientStyle};
-use canvas_traits::{RadialGradientStyle, RepetitionStyle, byte_swap};
+use canvas_traits::{RadialGradientStyle, RepetitionStyle, byte_swap, byte_swap_and_premultiply};
 use cssparser::Color as CSSColor;
 use cssparser::{Parser, RGBA};
 use dom::bindings::cell::DOMRefCell;
@@ -47,6 +47,7 @@ use std::str::FromStr;
 use std::{cmp, fmt};
 use unpremultiplytable::UNPREMULTIPLY_TABLE;
 use url::Url;
+use util::opts;
 
 #[must_root]
 #[derive(JSTraceable, Clone, HeapSizeOf)]
@@ -299,7 +300,14 @@ impl CanvasRenderingContext2D {
                     Some((mut data, size)) => {
                         // Pixels come from cache in BGRA order and drawImage expects RGBA so we
                         // have to swap the color values
-                        byte_swap(&mut data);
+                        if opts::get().use_webrender {
+                            // Webrender doesn't pre-multiply alpha when decoding
+                            // images, but canvas expects the images to be
+                            // pre-multiplied alpha.
+                            byte_swap_and_premultiply(&mut data);
+                        } else {
+                            byte_swap(&mut data);
+                        }
                         let size = Size2D::new(size.width as f64, size.height as f64);
                         (data, size)
                     },