Minor bit of cleanup in the ant scripts around the build src and target on javac, enabling use of Java 11 language features within processing itself. Note that the build is sustained on either 1.8 or 11 in terms of lanugage features but the code is no longer compatible with the Java 8 runtime due to backwards-incompatable changes made starting in Java 9.
Moves to Java11 and OpenJDK via AdoptOpenJDK within the processing4 train using a squash of sampottinger processing fork's java11 branch.
**Primary required changes:**
Some changes directly support OpenJFX / OpenJDK 11:
- Response to image loading changes caused by [JEP 320](https://openjdk.java.net/jeps/320)
- Use of jmodules as necessitated by [JEP 261](https://openjdk.java.net/jeps/261)
- Reponse to largely changed file paths caused by [JEP 220](https://openjdk.java.net/jeps/220).
- Modifications in build system related to AdoptOpenJDK and Java 11 which have a different naming structure for downloads.
- Allowing use of non-Oracle Java within internal Processing checks.
**Secondary required changes:**
There were some secondary required changes that impacted the usability of Processing after having moved to OpenJFX / OpenJDK 11:
- Removal of com.apple.eawt calls related to [JEP 272](http://openjdk.java.net/jeps/272)
- Response to HiDPI support on Windows and Linux in [JEP 263](https://openjdk.java.net/jeps/263)
- Removal of `java.ext.dirs`. Would be forced by [JEP 220](http://openjdk.java.net/jeps/220).
- Due to bugs on Windows, updated the JNA jars.
- Changes in downloader build tasks to support AdoptOpenJDK and OpenJFX.
- Updated org.eclipse.* / equinox jars.
- Some optimization around size of distribution.
- Update of AppBundler.
- Some changes in formulation of classpath and modifications in PreprocessingService given [JEP 261](https://openjdk.java.net/jeps/261).
**Incidental changes:**
This was (ahem) a bit of a larger PR with the above modifications. So, I wanted to introduce automated tests when possible and convenient along with a few changes for platform sustainability in order to support development:
- Addition of cross-building capability (!) made possible by AdoptOpenJDK.
- Addition of mockito for testing.
- Upgrade of junit.
- Addition of ant-contrib.
- Standardized nomenclature around JRE / JDK in `build/build.xml`
- Deduplication of code in `jre/build.xml`.
- Addition of JavaDoc in a few places.
- Some refactoring of PImage / Shape to support increased testing and readability in image manipulation code.
