demos: Make sure shaderClipDistance is available before requesting
demos: Make sure shaderClipDistance is available before requesting

Change-Id: Id0b58d54a553fce4914ddc1097f1e1c8ac3c337f

KhronosGroup/Vulkan-Tools,KhronosGroup/Vulkan-Tools,KhronosGroup/Vulkan-Tools,KhronosGroup/Vulkan-Tools