Update nanosvgrast.h

[Jony01]

Light speed up of nsvg__flattenCubicBez

[oehhar]

Dear Jony,
thank you for your proposed merge request.
To get it merged, it would be great, if you could explain your patch and what it does and which problems are solved.
As remarked before, only well prepared and explained patches are merged.

Thank you again,
Harald

[Jony01]

I think, when is it soo simple, it will be self explained from code.

It is speed up:
Remove unnecessary recursive Call at Last level.