Allow tilelive to pass in tile id. Prepares for delegation of dupe/id detection entirely to tilelive.

lib/mbtiles.js

@@ -529,7 +529,11 @@ MBTiles.prototype.putTile = function(z, x, y, data, callback) {
     // Flip Y coordinate because MBTiles files are TMS.
     y = (1 << z) - 1 - y;
 
-    var id = crypto.createHash('md5').update(data).digest('hex');
+    // Allow tilelive to pass us a data.id. If not, generate an md5
+    // from the image buffer data.
+    // @TODO refactor this such that it is entirely tilelive's responsibility
+    // to assign an ID to tiles and identify blanks. We're halfway there.
+    var id = data.id || crypto.createHash('md5').update(data).digest('hex');
     if (!this._tileCache[id]) {
         // This corresponds to the images table.
         this._tileCache[id] = data;