public synchronized int getNextId() {
        return ++id;
    }
Use this in the ParseContext to keep track of unique ids for rendered
images in embedded docs. This should be used for the full parse of
a main document and its embedded document.

This is different from RenderingState, which is used to track
rendering per file/per embedded doc.
tika-core/src/main/java/org/apache/tika/renderer/RenderingTracker.java