vcr: implement markers for vcr_present_mem_to_png()

This is a stupid simple naive implementation, but it does add
markers when enabled for mem->png (headless mode), e.g.:

`vmon --hertz 1 --markers 60 --headless --snapshots 3600`

Would give you minute markers in the borders of png snapshots
created every hour with 1hz samples (every column of pixels
represents a second, hence markers 60 gives you per-minute markers)

The current color used for the markers is not quite full
intensity yellow: {c0,c0,0}

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