Enforce positive frame rate cap

In `AppPreferences`, use the default value whenever an invalid frame rate cap is detected.

In the Preferences dialog, reject non-positive values with a `ParseException`. Though silent today, this is done in the
hopes that it will one day be signalled to the user somehow.

src/main/java/net/rptools/maptool/client/AppPreferences.java

@@ -1286,11 +1286,21 @@ public static void setMovementMetric(WalkerMetric metric) {
   }
 
   public static void setFrameRateCap(int cap) {
+    if (cap <= 0) {
+      // The provided value is invalid. Change to default instead.
+      cap = DEFAULT_FRAME_RATE_CAP;
+    }
     prefs.putInt(KEY_FRAME_RATE_CAP, cap);
   }
 
   public static int getFrameRateCap() {
-    return prefs.getInt(KEY_FRAME_RATE_CAP, DEFAULT_FRAME_RATE_CAP);
+    int result = prefs.getInt(KEY_FRAME_RATE_CAP, DEFAULT_FRAME_RATE_CAP);
+    if (result <= 0) {
+      // An invalid value is stored. Fix that.
+      result = DEFAULT_FRAME_RATE_CAP;
+      setFrameRateCap(result);
+    }
+    return result;
   }
 
   public static void setUpnpDiscoveryTimeout(int timeout) {

src/main/java/net/rptools/maptool/client/ui/preferencesdialog/PreferencesDialog.java

@@ -939,14 +939,20 @@ public void focusLost(FocusEvent e) {
               protected void storeNumericValue(Integer value) {
                 AppPreferences.setFrameRateCap(value);
 
+                // AppPreferences may have rejected the value, so read it back.
+                final var cap = AppPreferences.getFrameRateCap();
                 for (final var renderer : MapTool.getFrame().getZoneRenderers()) {
-                  renderer.setFrameRateCap(value);
+                  renderer.setFrameRateCap(cap);
                 }
               }
 
               @Override
               protected Integer convertString(String value) throws ParseException {
-                return StringUtil.parseInteger(value);
+                final var result = StringUtil.parseInteger(value);
+                if (result <= 0) {
+                  throw new ParseException("Frame rate cap must be positive", 0);
+                }
+                return result;
               }
             });