build: Update glslang to e4821e

This includes applying a patch to glslang to revert glslang
commit a5c33d, which fixes a texel fetch problem that many IHV
drivers are not ready to cope with yet.
diff --git a/update_external_sources.sh b/update_external_sources.sh
index 43992e2..f9f94ec 100755
--- a/update_external_sources.sh
+++ b/update_external_sources.sh
@@ -25,13 +25,14 @@
    cd $BASEDIR/glslang
    git fetch --all
    git checkout $GLSLANG_REVISION
-   # Special case for this particular revision:
-   # Pull in a patch that fixes a compilation issue with g++ 5.3
+   # Revert glslang a5c33d6ffb34ccede5b233bc724c907166b6e479
    # See https://github.com/KhronosGroup/Vulkan-LoaderAndValidationLayers/issues/681
-   if [ $GLSLANG_REVISION == "4678ca9dacfec7a084dbc69bbe568bdad6889f1b" ] ;
+   git diff-index --quiet HEAD | true
+   rc=${PIPESTATUS[0]}
+   if (( $rc == 0 ))
    then
-      git checkout $GLSLANG_REVISION -B temp1610
-      git cherry-pick 880bf36cacee1cfce7d5d94991eb18c9e2d59d39
+      echo "applying patch to revert glslang a5c33d"
+      git apply $BUILDDIR/glslang_revert_a5c33d.patch.txt
    fi
 }