plotters should accumulate input to avoid contention issue

[guruofquality]

Set the bins per display or points per wave plotter and the input sits on all the buffer resources waiting for more. Set the size back and it resumes plotting. We should in general address the issue of buffer requirements through the API, however in this case, accumulating to a render buffer at the display rate is very reasonable

• also see https://github.com/pothosware/pothos-library/issues/74

[guruofquality]

took a second look at how the buffer accumulator require() call worked: https://github.com/pothosware/pothos/commit/398538674a0f01771d78be152ee61b2587f8e25f