HTML Export sometimes doesn't include all rows of sequence alignment

Exporting the example file's first alignment view to HTML sometimes results in an incomplete export.

This turned out to be due to a race condition in the 'draw(..)' method which is called by both export routines, and also during AWT redraw events, eg. when a progress bar is being animated. Calling via the AWT redraw event loop reconfigured the SequenceCanvas' SequenceRenderer to the window manager's Graphics object, rather than the ImageExporter's graphics object.

Jalview 2.11.2 doesn't appear to be affected by this.