need to allocate new array when replacing buffer for any graphic features #1011

This basically means to create an "isolated buffer" when a new buffer is required for any feature. I didn't catch this for VertexColors in #974 , I should check the rest.

Encountered this when I was using an NDPositions of lines with colors allocated as the bottom subplot shows, but when I used a display window and slide around, I noticed the original buffer on NDPositionsProcessor.colors gets replaced as we slide around. This is because it was writing back to the NDPositionsProcessor.colors when setting the current display window. The fix is to make sure all replaced buffers are isolated.