Optimize Plotter Fills

[yuzawa-san]

fixes #1386

something about the polygon fills is not super efficient. i would imagine there is some sort of flood filling happening which is overkill since we know each pixels state. for this reason, i used drawPixel. i also go left to right row by row since that has the best performance. i also made the fill below a fillRect which is efficient for bulk regions.

UPDATE:
used vertical lines instead

[vladisslav2011]

On my machine this strategy is a lot slower, than drawing the fill with fillRect per point: ~20 ms per spectrogram @ 64k FFT vs ~9ms per spectrogram with fillRect. Unfilled spectrogram draws in ~7ms. I'm still using the old plotter (slightly patched to improve performance and add features) on linux though.
I'm getting ~100FPS without fill/75FPS with fill @ 64k FFT, 4k using fillRect strategy and 60FPS with fill using drawLines strategy.

[yuzawa-san]

i played around with fillRect and it appears to work nicely, so i'm putting that back. the only optimization i left was to do that bulk fill for the region below the lowest part of the polyline.

the bad performance before i believe was related to QT_THREAD_PARALLEL_FILLS being used, but something about the solid colors (instead of the alpha blending) seems to be avoiding that code path and making things faster. https://github.com/qt/qtbase/blob/b2e0ff4bca5d81205c8425cdc528e5c2a2988172/src/gui/painting/qdrawhelper.cpp#L3967

cc @markjfine

if this does not work, i'll just revert fully to whatever it was before.

[argilo]

The fill appears to be drawn one pixel too far to the right:

[yuzawa-san]

@argilo i also checked 2.17.5 and found that it was actually also off by a pixel there too, but i have fixed this. once again points in polygons can be in the middle of pixels, but the rectangles cannot it appears.

2.17.5:

this branch: