Process tracking for texture creation

Create and destroy a SceneJS process to track each texture layer creation.

This will allow texture creation to be monitored via SceneJS events, IE.

var nProcesses = 0;
SceneJS.addListener("process-created", function() {
   nProcesses++;
});
SceneJS.addListener("process-killed", function() {
   nProcesses--;
});

This is handy for scene loading progress indicators, logging etc. in applications built on SceneJS.