webp-lossless-bitstream-spec: fix dist mapping example
The distance code read from the bitstream is reduced by 1 before doing
the lookup. The prose describing the lookup was correct, the pseudocode
failed to subtract 1 and used x/y instead of xi/yi from the lookup.
@@ -764,14 +764,14 @@ The mapping between distance code `i` and the neighboring pixel offset
For example, distance code `1` indicates an offset of `(0, 1)` for the
neighboring pixel, that is, the pixel above the current pixel (0 pixel
difference in X-direction and 1 pixel difference in Y-direction). Similarly,
-distance code `3` indicates left-top pixel.
+distance code `3` indicates the left-top pixel.
The decoder can convert a distance code `i` to a scan-line order distance
`dist` as follows:
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
-(xi, yi) = distance_map[i]
-dist = x + y * xsize
+(xi, yi) = distance_map[i - 1]
+dist = xi + yi * xsize
if (dist < 1) {
dist = 1
}
