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.vs.glsl

@@ -0,0 +1,70 @@
+/* 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
+
+uniform mat4 uTransform;
+uniform vec4 uOffsets[32];
+uniform vec4 uClipRects[64];
+uniform mat4 uMatrixPalette[32];
+uniform vec2 uDirection;
+uniform vec4 uBlendParams;
+uniform vec4 uFilterParams;
+uniform float uDevicePixelRatio;
+uniform vec4 uTileParams[64];
+
+attribute vec3 aPosition;
+attribute vec4 aPositionRect;  // Width can be negative to flip horizontally (for border corners).
+attribute vec4 aColorRectTL;
+attribute vec4 aColorRectTR;
+attribute vec4 aColorRectBR;
+attribute vec4 aColorRectBL;
+attribute vec4 aColorTexCoordRectTop;
+attribute vec4 aColorTexCoordRectBottom;
+attribute vec4 aMaskTexCoordRectTop;
+attribute vec4 aMaskTexCoordRectBottom;
+attribute vec4 aBorderPosition;
+attribute vec4 aBorderRadii;
+attribute vec2 aSourceTextureSize;
+attribute vec2 aDestTextureSize;
+attribute float aBlurRadius;
+// x = matrix index; y = clip-in rect; z = clip-out rect; w = tile params index.
+//
+// A negative w value activates border corner mode. In this mode, the TR and BL colors are ignored,
+// the color of the top left corner applies to all vertices of the top left triangle, and the color
+// of the bottom right corner applies to all vertices of the bottom right triangle.
+attribute vec4 aMisc;
+
+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;
+
+int Bottom7Bits(int value) {
+    return value % 0x80;
+}
+
+bool IsBottomTriangle() {
+    // FIXME(pcwalton): No gl_VertexID in OpenGL ES 2. We'll need some extra data.
+    return false;
+}
+
+vec2 SnapToPixels(vec2 pos)
+{
+    // Snap the vertex to pixel position to guarantee correct texture
+    // sampling when using bilinear filtering.
+
+    // TODO(gw): ES2 doesn't have round(). Do we ever get negative coords here?
+    return floor(0.5 + pos * uDevicePixelRatio) / uDevicePixelRatio;
+}