slippy.PixelsToNative ignores pixels argument

[jchamberlain]

Problem

PixelsToNative should convert the accepted pixels number into projected/native units. Instead, it doesn't use the variable at all and merely returns the ratio of native to projected.

Solution

Refactor PixelsToNative to either:

1. Convert pixels, or
2. Not accept pixels (in which case, the function should be renamed to something like PixelsToNativeRatio).