Add flip vertical feature on decode

docs/decode.md

@@ -40,6 +40,7 @@ enum spng_decode_flags
 
     SPNG_DECODE_TRNS = 1, /* Apply transparency */
     SPNG_DECODE_GAMMA = 2, /* Apply gamma correction */
+    SPNG_DECODE_FLIP_Y = 16, /* Flip the image vertically */
     SPNG_DECODE_PROGRESSIVE = 256 /* Initialize for progressive reads */
 };
 ```

spng/spng.c

@@ -3216,6 +3216,11 @@ int spng_decode_image(spng_ctx *ctx, void *out, size_t len, int fmt, int flags)
     {
         size_t ioffset = ri->row_num * ctx->out_width;
 
+        if(flags & SPNG_DECODE_FLIP_Y)
+        {
+            ioffset = len - ctx->out_width - ioffset;
+        }
+
         ret = spng_decode_row(ctx, (unsigned char*)out + ioffset, ctx->out_width);
     }while(!ret);
 

spng/spng.h

@@ -172,6 +172,7 @@ enum spng_decode_flags
 
     SPNG_DECODE_TRNS = 1, /* Apply transparency */
     SPNG_DECODE_GAMMA = 2, /* Apply gamma correction */
+    SPNG_DECODE_FLIP_Y = 16, /* Flip the image vertically */
     SPNG_DECODE_PROGRESSIVE = 256 /* Initialize for progressive reads */
 };