Prevent Excessive Redraws (#4)

Only render the diagram if the definition changed.
This commit also adds a class for easier styling/formatting.

src/MermaidJS.Blazor/MermaidDiagram.razor

@@ -2,7 +2,7 @@
 
 @inject MermaidDiagramInterop _interop
 
-<div id="@Id"></div>
+<div id="@Id" class="mermaidjs-blazor-diagram"></div>
 @if (!string.IsNullOrWhiteSpace(errorMessage))
 {
     <p style="color:red;">@errorMessage</p>
@@ -32,6 +32,8 @@
     [Parameter]
     public EventCallback OnRender { get; set; }
 
+    string previousDefinition = string.Empty;
+
     public async ValueTask DisposeAsync()
     {
         await _interop.UnregisterComponent(this);
@@ -89,11 +91,13 @@
 
     protected override async Task OnParametersSetAsync()
     {
-        if (componentRef is null)
+        if (componentRef is null || Definition.Equals(previousDefinition))
         {
             return;
         }
 
+        previousDefinition = Definition;
+
         await BeginRender();
     }
 }