add pixel function and  floodFill function

## mat#pixel()

get pixel 
```javascript
mat.pixel(x,y);
``` 
return [b, g, r] exemple [0,80,255]

set pixel 
```javascript
mat.pixel(x,y, [b,g,r]);
``` 

## mat#floodFill()
```javascript
  mat.floodFill( { 
          seedPoint: [1,1], 
          newColor: [255,0,0] ,  
loDiff : [8,90,60], upDiff:[10,100,70]
  } );

```