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.
Would prefer all possible configurable variable of the function are either in parameters or as options with default.
Also help naming variables (downsizedBloomTextureFactor = 8 gives info about the 8 )
gives 
```javascript
createPostSceneBloom( sceneTexture, textureSize , options) {
if (!options) options = {};
if (!options.downsizedBloomTextureFactor ) options.downsizedBloomTextureFactor  = 8;
```