Properly enable QWebEngine GPU acceleration

Mentioned here: https://github.com/jellyfin/jellyfin-media-player/issues/10

I noticed the interface was lagging a lot when using a theme that has blurred backdrops (disabling the backdrops made it smooth again so I know that was the culprit). I knew it was because of a lack of hardware acceleration, and I stumbled across this issue and realized it was never actually enabled.

The difference in performance is night and day for me when compiled with this flag enabled.

src/main.cpp

@@ -81,7 +81,8 @@ void ShowLicenseInfo()
 
 /////////////////////////////////////////////////////////////////////////////////////////
 QStringList g_qtFlags = {
-  "--disable-web-security"
+  "--disable-web-security",
+  "--enable-gpu-rasterization"
 };
 
 /////////////////////////////////////////////////////////////////////////////////////////