Get image stream

Methods to get a merged resource image as a raw image stream (a string) which can be saved into a file or served on the fly by printing the result with the proper headers. Makes use of PHP's output buffering.

#### Included methods
- `getResultAsPNG($backgroundColor = null, $quality = 75)`
- `getResultAsJPG($backgroundColor = null, $quality = 75, $interlace = false)`
- `getResultAsGIF($backgroundColor = null)`