Update parser.js

Better than getting 0px width and height.
Specs say that when they are missing we should consider 100%, in our case 100% will be 100px because we don't have anything to compare the SVG to before adding the SVG into a canvas.
Otherwise we could take canvas widht and height.
Update parser.js

Added comment for future reference
Update parser.js

Added comment for future reference
Merge pull request #1624 from asturur/missing-width-or-height

Update parser.js