OffScreen - CaptureScreenshotAsync change to Scale default

- Previously when no Scale was specified, then the previous ChromiumWebBrowser scale was used.
- Now Viewport defaults to a scale of 1, so the behaviour has been changed so the scale passed as
  the param is always used.
- Test updated

This is a minor breaking change.

Resolves #4091

Author: amaitland

CefSharp.OffScreen/ChromiumWebBrowser.cs

@@ -550,6 +550,11 @@ public async Task<byte[]> CaptureScreenshotAsync(CaptureScreenshotFormat? format
                     return screenShot.Data;
                 }
 
+                if (viewport.Scale <= 0)
+                {
+                    throw new ArgumentException($"{nameof(viewport)}.{nameof(viewport.Scale)} must be greater than 0.");
+                }
+
                 //https://bitbucket.org/chromiumembedded/cef/issues/3103/offscreen-capture-screenshot-with-devtools
                 //CEF OSR mode doesn't set the size internally when CaptureScreenShot is called with a clip param specified, so
                 //we must manually resize our view if size is greater
@@ -563,19 +568,14 @@ public async Task<byte[]> CaptureScreenshotAsync(CaptureScreenshotFormat? format
                 {
                     newHeight = size.Height;
                 }
-                var newScale = viewport.Scale;
-                if (newScale == 0)
-                {
-                    newScale = deviceScaleFactor;
-                }
 
-                if ((int)newWidth > size.Width || (int)newHeight > size.Height || newScale != deviceScaleFactor)
+                if ((int)newWidth > size.Width || (int)newHeight > size.Height || viewport.Scale != deviceScaleFactor)
                 {
-                    await ResizeAsync((int)newWidth, (int)newHeight, (float)newScale).ConfigureAwait(continueOnCapturedContext:false);
+                    await ResizeAsync((int)newWidth, (int)newHeight, (float)viewport.Scale).ConfigureAwait(continueOnCapturedContext:false);
                 }
 
                 //Create a copy instead of modifying users object as we need to set Scale to 1
-                //as CEF doesn't support passing custom scale.
+                //as CEF doesn't support passing custom scale for OSR.
                 var clip = new Viewport
                 {
                     Height = viewport.Height,

CefSharp.Test/OffScreen/OffScreenBrowserBasicFacts.cs

@@ -808,7 +808,7 @@ public async Task CanCaptureScreenshotAsync()
                 }
 
 
-                var result3 = await browser.CaptureScreenshotAsync(viewport: new Viewport { Width = 100, Height = 200 });
+                var result3 = await browser.CaptureScreenshotAsync(viewport: new Viewport { Width = 100, Height = 200, Scale = 2 });
                 Assert.Equal(1466, browser.Size.Width);
                 Assert.Equal(968, browser.Size.Height);
                 Assert.Equal(2, browser.DeviceScaleFactor);

CefSharp.WinForms/ChromiumWebBrowser.cs

@@ -493,6 +493,11 @@ public async Task<byte[]> CaptureScreenshotAsync(CaptureScreenshotFormat format
             ThrowExceptionIfDisposed();
             ThrowExceptionIfBrowserNotInitialized();
 
+            if(viewPort != null && viewPort.Scale <= 0)
+            {
+                throw new ArgumentException($"{nameof(viewPort)}.{nameof(viewPort.Scale)} must be greater than 0.");
+            }
+
             using (var devToolsClient = browser.GetDevToolsClient())
             {
                 var screenShot = await devToolsClient.Page.CaptureScreenshotAsync(format, quality, viewPort, fromSurface, captureBeyondViewport).ConfigureAwait(continueOnCapturedContext: false);