Fix incorrectly parsed value of gid with horizontal flip flag on

[IoriBranford]

No description provided.

[baylej]

Hi, thank you for this,

What is the issue? I can't infer what it is from the contributed code.

Thanks!

[IoriBranford]

Gids in TMX files are unsigned int numbers, but atoi is only designed to read numbers in signed int range.
Ints nowadays have pretty much standardized to 32 bits, so a gid with the h-flip flag (the 32nd bit) is outside of signed int range. atoi doesn't handle this well and returns some incorrect value.