Implement overlay support in EGLRenderer

Fixes #405

app/shaders/egl_overlay.frag

@@ -0,0 +1,11 @@
+precision mediump float;
+uniform sampler2D uTexture;
+varying vec2 vTexCoord;
+
+void main() {
+    vec4 abgr = texture2D(uTexture, vTexCoord);
+
+    gl_FragColor = abgr;
+    gl_FragColor.r = abgr.b;
+    gl_FragColor.b = abgr.r;
+}

app/shaders/egl_overlay.vert

@@ -0,0 +1,8 @@
+attribute vec2 aPosition; // 2D: X,Y
+attribute vec2 aTexCoord;
+varying vec2 vTexCoord;
+
+void main() {
+    vTexCoord = aTexCoord;
+    gl_Position = vec4(aPosition, 0, 1);
+}