chore: support for tuning display frequency

frontends/web/ts/gb.ts

@@ -455,6 +455,7 @@ export class GameboyEmulator extends EmulatorLogic implements Emulator {
             ...(this.extraSettings?.debug ?? false
                 ? [
                       Feature.LoopMode,
+                      Feature.DisplayFrequency,
                       Feature.BootRomInfo,
                       Feature.RomTypeInfo,
                       Feature.Cyclerate,
@@ -592,6 +593,16 @@ export class GameboyEmulator extends EmulatorLogic implements Emulator {
         this.trigger("frequency", value);
     }
 
+    get displayFrequency(): number {
+        return this.visualFrequency;
+    }
+
+    set displayFrequency(value: number) {
+        value = Math.max(value, 0);
+        this.visualFrequency = value;
+        this.trigger("display-frequency", value);
+    }
+
     get frequencySpecs(): FrequencySpecs {
         return {
             unit: Frequency.MHz,