MeshMapFitPadding

How far the mesh map keeps each framed node from the edges, so the node's chip clears the map's own chrome.

Asymmetric because the chrome is: MapControlsOverlay reaches 72dp down from the top, the zoom pair reaches 72dp in from the lower trailing corner, and the logo and attribution run along the foot. A node chip is about 64 by 28dp, centred on its point, so each edge also leaves half a chip.