stb_image: Update comment

As per recent patches, we do support 16-bit PNMs.

stb_image.h

@@ -7321,7 +7321,6 @@ static int stbi__pic_info(stbi__context *s, int *x, int *y, int *comp)
 // Known limitations:
 //    Does not support comments in the header section
 //    Does not support ASCII image data (formats P2 and P3)
-//    Does not support 16-bit-per-channel
 
 #ifndef STBI_NO_PNM