Bug in SelectColumnLayer.prepareSourcesForParallelPopulation

[rcaudy]

A recent fix to prepareForParallelPopulation for the Immutable*ArraySource implementations exposed that we're passing the wrong RowSequence when flattenedResult. We can also be a little more optimal regarding modifies in the common case.

java.lang.UnsupportedOperationException: (No message provided)	
at io.deephaven.engine.table.impl.sources.immutable.Immutable2DByteArraySource.ensureCapacity(Immutable2DByteArraySource.java:123)	
at io.deephaven.engine.table.impl.sources.immutable.Immutable2DByteArraySource.prepareForParallelPopulation(Immutable2DByteArraySource.java:342)	
at io.deephaven.engine.table.impl.select.analyzers.SelectColumnLayer.prepareSourcesForParallelPopulation(SelectColumnLayer.java:561)	
at io.deephaven.engine.table.impl.select.analyzers.SelectColumnLayer.prepareParallelUpdate(SelectColumnLayer.java:219)	
at io.deephaven.engine.table.impl.select.analyzers.SelectColumnLayer$1.lambda$onAllRequiredColumnsCompleted$0(SelectColumnLayer.java:196)	
at io.deephaven.engine.table.impl.util.OperationInitializationPoolJobScheduler.lambda$submit$0(OperationInitializationPoolJobScheduler.java:27)	
at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:539)	
at java.util.concurrent.FutureTask.run(FutureTask.java:264)	
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1136)	
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:635)	
at io.deephaven.engine.table.impl.OperationInitializationThreadPool$1.lambda$newThread$0(OperationInitializationThreadPool.java:50)	
at java.lang.Thread.run(Thread.java:833)