Fix image sampling when width x height < than 300x250

When qr_code div element's width x height values are lower than 300 x
250, sampling process uses fixed width x height values (307 x 250)
disregarding custom values defined by user. This fix removes hard-coded
width x height values and uses variables instead.

src/html5-qrcode.js

@@ -25,7 +25,7 @@
 
                 var scan = function() {
                     if (localMediaStream) {
-                        context.drawImage(video, 0, 0, 307, 250);
+                        context.drawImage(video, 0, 0, width, height);
 
                         try {
                             qrcode.decode();