Fix macOS mouse position reporting

Under SDL2, with HighDPI enabled, we end up with two coordinate
systems for our window... there's the pixel coordinates which we
draw in, and are available via SDL_GetWindowSizeInPixels(), and
there's the "screen" coordinates, which is an underlying source
size that is scaled from, and is available via SDL_GetWindowSize().

Mouse events are in the screen coordinates, drawing is in pixel
coordinates.  This commit converts mouse event positions to pixel
coordinates before converting further to text coordinates. :(

A specual thanks to u/ten-oh-four for working to track this down
with me, they ran multiple experimental builds and collected logs
which allowed to to be tracked down.

Fixes issue 155