Add PWM brightness control for the M5ToughC5 backlight - #250
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The backlight was previously on/off only, so setBrightness had no dimming effect. Drive it with the IO expander PWM channel in the same way as the ChainCaptain, with a gamma 2.0 curve so the perceived brightness follows the setting.
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The M5ToughC5 backlight control was on/off only, so
setBrightness()had no dimming effect between 1 and 255.This drives the backlight with the I/O expander's PWM channel, the same approach already used for the ChainCaptain (and the PaperMono on its PMIC): 1kHz PWM, 12-bit duty, with a gamma 2.0 curve so the perceived brightness follows the setting. Zero still turns the backlight fully off.
Verified on an M5ToughC5 with a brightness slider sweeping the full range.