Structure viewer image export can hang when run from command line

Creating images from Jmol views of structures via the command line can result in deadlocks or 'near deadlocks' when --gui is passed.

This command more-or-less-reliably deadlocks on OSX/12.6.6
java -jar build/libs/jalview-all-TEST-j11.jar --gui --open=./examples/test_fab41.result/sample.a2m --structure=./examples/test_fab41.result/test_fab41_unrelaxed_rank_1_model_3.pdb --structureimage=test/jalview/bin/argparser/testfiles/structureimage1.png --open=./examples/test_fab41.result/sample.a2m --structure=./examples/test_fab41.result/test_fab41_unrelaxed_rank_1_model_3.pdb --structureimage=test/jalview/bin/argparser/testfiles/structureimage2.png --structureimagescale=1.5 --open=./examples/test_fab41.result/sample.a2m --structure=./examples/test_fab41.result/test_fab41_unrelaxed_rank_1_model_3.pdb --structureimage=test/jalview/bin/argparser/testfiles/structureimage3.png --structureimagescale=2.0