Mechanism to clear waterfall when switching modulations.

firmware/application/ui_receiver.cpp

@@ -567,6 +567,8 @@ void ReceiverView::on_modulation_changed(int32_t modulation) {
 		receiver_model.set_baseband_bandwidth(1750000);
 		break;
 	}
+
+	waterfall.clear();
 }
 
 void ReceiverView::on_show_options_frequency() {

firmware/application/ui_spectrum.hpp

@@ -75,6 +75,12 @@ public:
 		draw_frequency_ticks(painter, r);
 	}
 
+	void clear() {
+		spectrum_sampling_rate = 0;
+		spectrum_bins = 0;
+		set_dirty();
+	}
+
 private:
 	static constexpr Dim filter_band_height = 4;
 
@@ -197,6 +203,13 @@ public:
 		(void)painter;
 	}
 
+	void clear() {
+		display.fill_rectangle(
+			screen_rect(),
+			Color::black()
+		);
+	}
+
 	void on_channel_spectrum(
 		const ChannelSpectrum& spectrum
 	) {
@@ -260,6 +273,11 @@ public:
 		(void)painter;
 	}
 
+	void clear() {
+		waterfall_view.clear();
+		frequency_scale.clear();
+	}
+
 private:
 	WaterfallView waterfall_view;
 	FrequencyScale frequency_scale;