make PreviewView smaller.

The 160x120 preview size is a bad choice, because it makes XRoger crash
in an infinite recursion when trying to draw bezier arcs with custom
code (instead of using the BeAPI for that).

Use 120x90, just like BeOS, to maximise compatibility and avoid such
problems.

Fixes #10601.

src/preferences/screensaver/PreviewView.cpp

@@ -130,7 +130,7 @@ PreviewView::AddPreview()
 
 	float aspectRatio = 4.0f / 3.0f;
 		// 4:3 monitor
-	float previewWidth = 160.0f;
+	float previewWidth = 120.0f;
 	float previewHeight = ceilf(previewWidth / aspectRatio);
 
 	fSaverView->SetExplicitSize(BSize(previewWidth, previewHeight));