In case VolatileImage uses IndexColorModel, the colours in the image may not match the ones we expect. We can use BufferedImage with the same colour model as the golden image to validate VolatileImage looks as expected.

The affected tests:
java/awt/BasicStroke/DashOffset.java
java/awt/BasicStroke/DashScaleMinWidth.java
java/awt/BasicStroke/DashZeroWidth.java


It was suggested in the code review for JDK-8218682 and JDK-8198411:
http://mail.openjdk.java.net/pipermail/2d-dev/2019-February/009850.html
http://mail.openjdk.java.net/pipermail/2d-dev/2019-February/009852.html
http://mail.openjdk.java.net/pipermail/2d-dev/2019-February/009854.html