Add "missing" C API functions.

[tjko]

I noticed that some of the C API functions were "missing" (declared in PNGdec.h but not found in png.inl)

int PNG_decode(PNGIMAGE *pPNG, void *pUser, int iOptions);
int PNG_getBpp(PNGIMAGE *pPNG);
int PNG_getPixelType(PNGIMAGE *pPNG);
int PNG_hasAlpha(PNGIMAGE *pPNG);
int PNG_isInterlaced(PNGIMAGE *pPNG);
void PNG_setBuffer(PNGIMAGE *pPNG, uint8_t *pBuffer);

This patch implements these, so that library usable from C...