Minor issues in the new plotter

[argilo]

The new plotter (introduced in #1218, and soon to be released in version 2.17) has a few minor issues:

• There is a row of black pixels on the left side of the waterfall (except in the most recent FFT).
• At some zoom levels, there is an empty pixel at the right hand side of the plot and waterfall.
• Dragging the mouse over the plotter or waterfall to change the demod frequency causes the peak/max/min display to reset.
• When the DSP is paused, zooming in or out on the frequency axis causes the peak/max/min display to be one "frame" behind.
• Parts of the overlay (especially the center line and demod line) can obscure thin peaks in the FFT plot.
• In the band plan, labels for wide bands are often displayed off-screen.
• X coordinates are sometimes slightly misaligned due to remnants of integer-based pixel calculations, for instance in xFromFreq.
• Min hold sometimes doesn't work when the DSP is first started, because rx_fft_c::get_fft_data returns zeroes if no samples have arrived yet from the SDR hardware.

Since the issues are minor, they don't necessarily need to be fixed prior to the 2.17 release.

[argilo]

The 2.17 release is out, and most of these issues have been fixed. I'll split the coordinate issue off.