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/es2_common.fs.glsl

@@ -0,0 +1,42 @@
+/* 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/. */
+
+#version 110
+
+#define SERVO_ES2
+
+precision highp float;
+
+uniform sampler2D sDiffuse;
+uniform sampler2D sMask;
+uniform vec4 uBlendParams;
+uniform vec4 uAtlasParams;
+uniform vec2 uDirection;
+uniform vec4 uFilterParams;
+
+varying vec2 vPosition;
+varying vec4 vColor;
+varying vec2 vColorTexCoord;
+varying vec2 vMaskTexCoord;
+varying vec4 vBorderPosition;
+varying vec4 vBorderRadii;
+varying vec2 vDestTextureSize;
+varying vec2 vSourceTextureSize;
+varying float vBlurRadius;
+varying vec4 vTileParams;
+varying vec4 vClipInRect;
+varying vec4 vClipOutRect;
+
+vec4 Texture(sampler2D sampler, vec2 texCoord) {
+    return texture2D(sampler, texCoord);
+}
+
+float GetAlphaFromMask(vec4 mask) {
+    return mask.a;
+}
+
+void SetFragColor(vec4 color) {
+    gl_FragColor = color;
+}
+