Proposal: improve spectrum drawing speed

[KI3P]

The current method of drawing the spectrum in MainBoard_DisplayHome.cpp, function ShowSpectrum(), draws a vertical yellow line for each bin of the spectrum. Before doing so it overwrites the previous spectrum with a black line. This requires two calls to tft.drawLine for each spectral bin.

An alternative approach is proposed in this forum message: use LAYER2 to prepare the spectrum image before block-transferring it to LAYER 1. Greg KF5N has partial proof-of-concept implementation of this in his code to the T41 -- see file T41EEE/Display.cpp at https://github.com/Greg-R/T41EEE/tree/layer_shuffle

Rewrite ShowSpectrum() to use the block transfer method. This might require changes to ShowBandwidth() and DrawBandWidthIndicatorBar(), which also draw information in that portion of the pane.