sns: simple filters with capacity of 1

code/espurna/filters/LastFilter.h

@@ -13,6 +13,10 @@ public:
         _value = value;
     }
 
+    size_t capacity() const override {
+        return 1;
+    }
+
     void reset() override {
         _value = 0;
     }

code/espurna/filters/MaxFilter.h

@@ -15,6 +15,10 @@ public:
         _value = std::max(value, _value);
     }
 
+    size_t capacity() const override {
+        return 1;
+    }
+
     void reset() override {
         _value = 0;
     }