Fix screen_to_map for flare-game hdcore.

In the hdcore mod UNITS_PER_PIXEL_X == 1, so it will produce wrong results
when calculating UNITS_PER_PIXEL_X /2 and using it for further calculations

Instead move the /2 to be done at the srcx and srcy variables, which
have a larger range, so the calculation error will be smaller.

Fixes #584