On-Screen Frame Counter for vkcube

[andrew-lunarg]

Downstream tools such as layers which display overlays are able to paint the current frame number but it would be useful at least as a check on them or to simplify the layer stack for the vkcube app to print it's idea of the current frame number into the framebuffer. This would mostly be of interest as a visual confirmation for authors of tools which are able to take actions over specific frame ranges such as trace capture and replay that those tools selected the correct frames to operate on. Of course the big issue would be, does the frame count start at zero or one?