Changed preserveDrawingBuffer to be an option

jsmpg.js

 	this.wantsToPlay = this.autoplay;
 	this.loop = !!opts.loop;
 	this.seekable = !!opts.seekable;
+  	this.preserveDrawingBuffer = !!opts.preserveDrawingBuffer;
 	this.externalLoadCallback = opts.onload || null;
 	this.externalDecodeCallback = opts.ondecodeframe || null;
 	this.externalFinishedCallback = opts.onfinished || null;
 
 	// attempt to get a webgl context
 	try {
-		var options = { preserveDrawingBuffer: true };
+    		var options = { preserveDrawingBuffer: this.preserveDrawingBuffer };
 		gl = this.gl = this.canvas.getContext('webgl', options) || this.canvas.getContext('experimental-webgl', options);
 	} catch (e) {
 		return false;