Clear background when using a theme with transparent background

This partially reverts 6ffd0f2e92

While we do not support transparent themes per se, we still use the alpha colors and thus would previously fill the background using alpha, into a layer that we hadn’t previously cleared.

Frameworks/OakTextView/src/OakTextView.mm

@@ -1210,6 +1210,12 @@ doScroll:
 		return;
 	}
 
+	if(self.theme->is_transparent())
+	{
+		[NSColor.clearColor set];
+		NSRectFill(aRect);
+	}
+
 	CGContextRef context = NSGraphicsContext.currentContext.CGContext;
 	if(!self.antiAlias)
 		CGContextSetShouldAntialias(context, false);