use rpng_nbio.c now - try to cover PNG_CHUNK_PLTE as well

Makefile.common

@@ -570,7 +570,7 @@ endif
 
 ifeq ($(HAVE_ZLIB), 1)
    ZLIB_OBJS =	decompress/zip_support.o 
-   OBJ += libretro-common/formats/png/rpng.o file_extract.o
+   OBJ += libretro-common/formats/png/rpng_nbio.o file_extract.o
    OBJ += $(ZLIB_OBJS)
    DEFINES += -DHAVE_ZLIB
    HAVE_COMPRESSION = 1 

libretro-common/formats/png/rpng_nbio.c

@@ -660,6 +660,8 @@ bool rpng_load_image_argb_iterate(uint8_t *buf,
             if (entries > 256)
                return false;
 
+            buf += 8;
+
             for (i = 0; i < entries; i++)
                palette[i] = buf[i];