Set image 'opacity' attribute in drawImage method

[kinimesi]

Set opacity attribute of images. Right now, the opacity is always 1 in converted SVG files, try the following:

ctx.globalAlpha = 0.5;
var img = new Image();
img.src = "https://www.w3schools.com/tags/img_the_scream.jpg"

ctx.drawImage(img, 0, 0);

[mudcube]

This pull request works. @webspinner Any chance of getting this merged in? Thanks!