Issue #2365469: Fix ob_flush notice.
Issue #2365469: Make dynamic asset handling an option on the stack.
Issue #2365469: Initialize the renderStack within the static class for performance reasons.
Issue #2365469: Add callbacks for drupal_add_* functions to static class.
Issue #2365469: Add asset helper functions.
Issue #2365469: Fix tests.
Issue #2365469: Fix test coverage.
Issue #2365469: Use deep merging for adding #cache properties.