Added a fix for X11 / Cinnamon window popups and maximize button not visible.

ptlinux/native/gamescope-wayland-letterbox-backing.patch

@@ -0,0 +1,21 @@
+From: PT Linux Launcher maintainers
+Subject: Use a backing surface for letter-boxing
+
+Backport upstream commit 66450376038864700f9b4721343b4d15b29c4198 so
+native Wayland presentation covers the complete configured output when the
+game layer is unity-scaled with a non-zero letterbox offset.
+
+--- a/src/Backends/WaylandBackend.cpp
++++ b/src/Backends/WaylandBackend.cpp
+@@ -1081,7 +1081,10 @@ namespace gamescope
+                 bool bNeedsBacking = true;
+                 if ( pFrameInfo->layerCount >= 1 )
+                 {
+-                    if ( pFrameInfo->layers[0].isScreenSize() && !pFrameInfo->layers[0].hasAlpha() )
++                    if ( pFrameInfo->layers[0].isScreenSize() &&
++                         close_enough( pFrameInfo->layers[0].offset.x, 0.0f ) &&
++                         close_enough( pFrameInfo->layers[0].offset.y, 0.0f ) &&
++                         !pFrameInfo->layers[0].hasAlpha() )
+                         bNeedsBacking = false;
+                 }
+