Work-around iOS driver bug: stencil initial value must be explicitly set to zero.

Even though the doc says its default value is already zero, without setting
it, stencil operations won't work properly on iOS devices (except emulator).

Author: kakashidinho

Diff for: src/libANGLE/renderer/metal/mtl_command_buffer.mm

@@ -1153,6 +1153,10 @@ void PopDebugGroupCmd(id<MTLRenderCommandEncoder> encoder, IntermediateCommandSt
         // Verify that it was created successfully
         ASSERT(get());
 
+        // Work-around driver bug on iOS devices: stencil must be explicitly set to zero
+        // even if the doc says the default value is already zero.
+        [metalCmdEncoder setStencilReferenceValue:0];
+
         while (mCommands.good())
         {
             auto cmdType               = mCommands.fetch<CmdType>();