An abstract projection class and child mercprojection class

I'm not clear on the purpose of the "pixel" functions, the results of any pixel measurements here aren't meaningful without knowing the display size (which only the view module would have, at the moment). Even then, "pixels" are a problematic unit of measurement on mobile devices where native pixel doubling or tripling is common. Measuring tiles in pixels is a common practice on legacy raster tile maps, but I don't know that it has a use for us.