Move _IMPLEMENTATION outside of _H

[ysoftware]

In my project multiple files want to include nanosvg.h.
I also like to add all #define *_IMPLEMENTATION at the end of main.c, just to keep things in one place.

My usage:

#include "example.h" // includes nanosvg.h

...

int main(void) { ... }

#define NANOSVG_IMPLEMENTATION
#include "nanosvg.h"

#define NANOSVGRAST_IMPLEMENTATION
#include "nanosvgrast.h"

Currently this is not possible because if the implementation is not included the first time, it will never be included, since it's declared inside of NANOSVG_H guard. With the new change, you can do this.

One possible issue I see is that if the user declares NANOSVG_IMPLEMENTATION early, then all successive includes will produce an error (since implementation will keep getting added multiple times). I feel like this is good and should not be solved by the library, since it is not trying to hide information from the user and gives them proper understanding of what is going on. It is also easily solvable with an #undef. I don't insist on this decision though, if you disagree.