jalview.bin.CommandsTest.headlessOrGuiImageOutputTest produces different images for --gui and --headless when using certain .jalview_properties

**RECORDING FOR POSTERITY** [~jprocter] confirmed that jalview.bin.CommandsTest.headlessOrGuiImageOutputTest was apparently failing consistently on both the build server and locally on OSX/Apple Silicon.

Specific execution also showed it to fail:
gradle test --tests jalview.bin.CommandsTest.headlessOrGuiImageOutputTest

however interactive execution via eclipse did not fail. Executing a clean and rebuild seemed to fix the problem.

> Task :testTask3
Picked up JAVA_TOOL_OPTIONS: -Dapple.awt.UIElement=true

Gradle suite > Gradle test > jalview.bin.CommandsTest.headlessOrGuiImageOutputTest[0]([Ljava.lang.String;@106faf11, [Ljava.lang.String;@70f43b45) FAILED
    java.lang.AssertionError: New file test/jalview/bin/argparser/testfiles/test-al-pae-ss-nogui.png (actual size) not same as last file's size test/jalview/bin/argparser/testfiles/test-al-pae-ss-gui.png expected [153925] but found [156481]
        at org.testng.Assert.fail(Assert.java:94)
        at org.testng.Assert.failNotEquals(Assert.java:496)
        at org.testng.Assert.assertEquals(Assert.java:125)
        at org.testng.Assert.assertEquals(Assert.java:267)
        at jalview.bin.CommandsTest.verifyOrderedFileSet(CommandsTest.java:304)
        at jalview.bin.CommandsTest.headlessOrGuiImageOutputTest(CommandsTest.java:233)