Add WebRender integration to Servo.

WebRender is an experimental GPU accelerated rendering backend for Servo.

The WebRender backend can be specified by running Servo with the -w option (otherwise the default rendering backend will be used).

WebRender has many bugs, and missing features - but it is usable to browse most websites - please report any WebRender specific rendering bugs you encounter!

resources/shaders/quad.fs.glsl

@@ -0,0 +1,38 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+// GLSL point in rect test.
+// See: https://stackoverflow.com/questions/12751080/glsl-point-inside-box-test
+bool PointInRect(vec2 p, vec2 p0, vec2 p1)
+{
+    vec2 s = step(p0, p) - step(p1, p);
+    return s.x * s.y != 0.0;
+}
+
+void main(void)
+{
+    // Clip out.
+    if (PointInRect(vPosition, vClipOutRect.xy, vClipOutRect.zw)) {
+        discard;
+    }
+
+    // Clip in.
+    if (!PointInRect(vPosition, vClipInRect.xy, vClipInRect.zw)) {
+        discard;
+    }
+
+    // Apply image tiling parameters (offset and scale) to color UVs.
+    vec2 colorTexCoord = vTileParams.xy + fract(vColorTexCoord.xy) * vTileParams.zw;
+    vec2 maskTexCoord = vMaskTexCoord.xy;
+
+    // Fetch the diffuse and mask texels.
+    vec4 diffuse = Texture(sDiffuse, colorTexCoord);
+    vec4 mask = Texture(sMask, maskTexCoord);
+
+    // Extract alpha from the mask (component depends on platform)
+    float alpha = GetAlphaFromMask(mask);
+
+    // Write the final fragment color.
+    SetFragColor(diffuse * vec4(vColor.rgb, vColor.a * alpha));
+}