Copy shaders from WR repo.

resources/shaders/ps_blend.fs.glsl

@@ -0,0 +1,10 @@
+/* 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/. */
+
+uniform sampler2D sCache;
+
+void main(void) {
+    vec4 color = texture(sCache, vUv);
+    oFragColor = vec4(color.rgb, color.a * vOpacity);
+}