broadway: Refcount textures

We want to delay some rendering, and to make that safe we need to correctly
refcount the use of blob uris for the textures so that we don't unref
it while something is scheduled to use it.

gdk/broadway/broadway.js

@@ -128,6 +128,27 @@ function getButtonMask (button) {
     return 0;
 }
 
+function Texture(id, data) {
+    var blob = new Blob([data],{type: "image/png"});
+    this.url = window.URL.createObjectURL(blob);
+    this.refcount = 1;
+    this.id = id;
+    textures[id] = this;
+}
+
+Texture.prototype.ref = function() {
+    this.refcount += 1;
+    return this;
+}
+
+Texture.prototype.unref = function() {
+    this.refcount -= 1;
+    if (this.refcount == 0) {
+        window.URL.revokeObjectURL(this.url);
+        delete textures[this.id];
+    }
+}
+
 function sendConfigureNotify(surface)
 {
     sendInput("w", [surface.id, surface.x, surface.y, surface.width, surface.height]);
@@ -499,8 +520,11 @@ SwapNodes.prototype.insertNode = function(parent, posInParent, oldNode)
             image.height = rect.height;
             image.style["position"] = "absolute";
             set_rect_style(image, rect);
-            var texture_url = textures[texture_id];
-            image.src = texture_url;
+            var texture = textures[texture_id];
+            image.src = texture.url;
+            texture.ref();
+            // Unref blob url when loaded
+            image.onload = function() { texture.unref(); };
             newNode = image;
         }
         break;
@@ -789,16 +813,12 @@ function cmdSurfaceSetNodes(id, node_data)
 
 function cmdUploadTexture(id, data)
 {
-    var blob = new Blob([data],{type: "image/png"});
-    var url = window.URL.createObjectURL(blob);
-    textures[id] = url;
+    new Texture (id, data); // Stores a ref in textures
 }
 
 function cmdReleaseTexture(id)
 {
-    var url = textures[id];
-    window.URL.revokeObjectURL(url);
-    delete textures[id];
+    textures[id].unref();
 }
 
 function cmdGrabPointer(id, ownerEvents)