qt55.qtbase: upstream patch for backing store issue

The included patch from upstream fixes the issue described here:
https://bugreports.qt.io/browse/QTBUG-48321
The backing store of certain widgets was being improperly invalidated,
leading to display bugs in, e.g. VLC.

This patch is included in Qt 5.6, so we should remove it when we
upgrade.

pkgs/development/libraries/qt-5/5.5/qtbase/default.nix

@@ -26,6 +26,12 @@
 let
   inherit (srcs.qt5) version;
   system-x86_64 = lib.elem stdenv.system lib.platforms.x86_64;
+
+  dontInvalidateBacking = fetchurl {
+    url = "https://codereview.qt-project.org/gitweb?p=qt/qtbase.git;a=patch;h=0f68f8920573cdce1729a285a92ac8582df32841;hp=24c50f8dcf7fa61ac3c3d4d6295c259a104a2b8c";
+    name = "qtbug-48321-dont-invalidate-backing-store.patch";
+    sha256 = "07vnndmvri73psz0nrs2hg0zw2i4b1k1igy2al6kwjbp7d5xpglr";
+  };
 in
 
 stdenv.mkDerivation {
@@ -51,6 +57,10 @@ stdenv.mkDerivation {
 
   postPatch =
     ''
+      cd qtbase
+      patch -p1 <${dontInvalidateBacking}
+      cd ..
+
       substituteInPlace configure --replace /bin/pwd pwd
       substituteInPlace qtbase/configure --replace /bin/pwd pwd
       substituteInPlace qtbase/src/corelib/global/global.pri --replace /bin/ls ${coreutils}/bin/ls