support `find`ing the key for a position.

js/pixl/public/trixl.html

@@ -222,7 +222,7 @@
			[[+0.0, +3.0, 2], {color: [0.5, 0.5, 0.5, 1]}]
		]);
		trixl.world.remove = function(pos) {
		trixl.world.find = function(pos) {
			var key = null;
			for(var k of trixl.world.keys()) {
				if (k[0] === pos[0] && k[1] === pos[1] && k[2] === pos[2]) {
@@ -230,7 +230,11 @@
					break;
				}
			}
			return trixl.world.delete(key);
			return key;
		}
		trixl.world.remove = function(pos) {
			return trixl.world.delete(trixl.world.find(pos));
		}
		trixl.world.dynamic = [];