exo: Add support for alpha_compositing interface to wayland bindings.

ui/compositor/layer.cc

Index: ui/compositor/layer.cc
diff --git a/ui/compositor/layer.cc b/ui/compositor/layer.cc
index 8c8a29195c529640c22758434febfdc5e3b7ba7f..eb03c430aec9977e221a280f0fb1bc7a590ecfab 100644
--- a/ui/compositor/layer.cc
+++ b/ui/compositor/layer.cc
@@ -566,6 +566,11 @@ bool Layer::TextureFlipped() const {
   return texture_layer_->flipped();
 }
 
+void Layer::SetTextureAlpha(float alpha) {
+  DCHECK(texture_layer_.get());
+  texture_layer_->SetVertexOpacity(alpha, alpha, alpha, alpha);
+}
+
 void Layer::SetShowSurface(
     cc::SurfaceId surface_id,
     const cc::SurfaceLayer::SatisfyCallback& satisfy_callback,