firequalizer15.lua: Add a function to turn the equalizer + Smoother equalizer text, minor changes

[DaMaNic2022]

1. Add a function to turn the equalizer on and off, similar to https://github.com/mpv-player/mpv/blob/master/TOOLS/lua/acompressor.lua
2. Smoother equalizer text, minor changes: local freq_label = { eq0 = "# 0 Hz: ", eq1 = "# 65 Hz: ", eq2 = "# 157 Hz: ", eq3 = "# 288 Hz: ", eq4 = "# 472 Hz: ", eq5 = "# 733 Hz: ", eq6 = "# 1.1k Hz: ", eq7 = "# 1.6k Hz: ", eq8 = "# 2.4k Hz: ", eq9 = "# 3.4k Hz: ", eq10 = "# 4.9k Hz: ", eq11 = "# 7k Hz: ", eq12 = "# 10k Hz: ", eq13 = "# 14k Hz: ", eq14 = "# 20k Hz: " } ... local fontfamily = "monospace" ... local function gain_line(x) ... x = x + 10 ... local function show_osd_ass() ... str = str .. pval .. freq_label[eq(x)] .. string.format("%+05.1f", gain_table[eq(x)]*0.1) .. " dB |" .. gain_line(x) .. "\n"