Added example composer example with stitching, vignetting and bloom examples

Added a bloom shader. There are 2 implementations for it but i didn't manage to setup a proper runtime switch in time so here they are.
Reading those blogpost didn't help finding the reasonning behind those magic formulae, and it bothers me.
for me, direct from http://en.wikipedia.org/wiki/Luminance_(relative)
```javascript
// get approximative luminance from an RGB value
float CalcLuminance(vec3 color)
{
    return max(dot(color, vec3(0.299, 0.587, 0.114)), 0.0001);
}
```
Now with that luminance value, you can filter out (save only) bright pixel, and that gives a "brightfilterpass"
And that brightexture, once downsized can be used as an averageLuminance getter...
```javascript
// get Enhanced averaged luminance from downsized brighpass filter texture
float CalcLuminance(float lum)
{
    return exp(lum);
}
```