adjust padding offset

PR for #416

This is in current build:
```javascript
x = temp[0] * (tile + paddingX);
y = temp[1] * (tileh + paddingY);
```
However the author of the issue suggests:
```javascript
x = temp[0] * (tile + paddingX) + paddingX;
y = temp[1] * (tileh + paddingY) + paddingY;
```

Look at the example tileset in the referenced issue. The tileset has padding before the first tile, so his solution is correct:
let `temp[0] = 0`, then `x = 0*(...) = 0`. Instead `x = 0*(...) + paddingX = paddingX` is required.
left `temp[0] = 1`, then `x = 1*(tileWidth + paddingX) `. Instead  `x = 1*(tileWidth + paddingX) + paddingX` is required (2 paddings required for 2nd x tile).

I have tested it with the example tileset and in the current build the black border is indeed visible. With this fix the black border is not visible. The question is whether __every__ spriteset has padding like this?