Fix marking all cube faces as clean after update.

Change-Id: I85886ea83f4d748dce98d79acff204bf855da1ad
Reviewed-on: https://swiftshader-review.googlesource.com/17569
Tested-by: Nicolas Capens <nicolascapens@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>
diff --git a/src/OpenGL/libGLESv2/Texture.cpp b/src/OpenGL/libGLESv2/Texture.cpp
index 364fef8..dc0e807 100644
--- a/src/OpenGL/libGLESv2/Texture.cpp
+++ b/src/OpenGL/libGLESv2/Texture.cpp
@@ -1200,7 +1200,7 @@
 	posZ->markContentsClean();
 	negX->markContentsClean();
 	negY->markContentsClean();
-	negY->markContentsClean();
+	negZ->markContentsClean();
 }
 
 bool TextureCubeMap::isCompressed(GLenum target, GLint level) const