New Vulkan example: Indirect drawing

[Stefan]

Sascha Willems has added another example to his open source C++ Vulkan examples. The new one is about indirect drawing (including multi draw if supported). Contrary to their non-direct counterparts, the indirect drawing commands in Vulkan take their draw calls from a buffer that is ideally stored in device local memory. So instead of running single draw commands that get their index base, index count and instancing numbers passed by the host upon getting called, the indirect commands are backed by a buffer that stores an arbitrary number of draw commands.