Enable debugPrintf example on macOS using validation layer

[SRSaunders]

This small PR adds basic support for the debugprintf example on macOS. The shader debugPrintfEXT capability is not native on macOS/MoltenVK, but can be provided by VK_LAYER_KHRONOS_validation instead. This change forces validation on for this example and enables the layer's debugPrintfEXT feature when running on macOS. Without this change, the example's shaders will not cross compile to MSL and will not run on macOS.

Also, since RenderDoc is not available on macOS, and native graphics debuggers are Metal-centric (not Vulkan), this PR also enables the validation layer's printf to stdout feature which redirects output to the console window. Since the example is free running, you will see a continuous stream of printf output to the console, and periodically will observe a buffer overflow warning from the validation layer. This is a result of the layer using a fixed size internal printf buffer and can be safely ignored.

Fixes #1133