Fix text glitche

[asturur]

What this commit do:

• Resolve text decoration issue for some textAlign values
• Remove a couple of canvas.save and canvas.restore couples
• Solve a regression: when text is justified decoration and backgroundcolor are not.
• do not justify text if a line contains just one word

[asturur]

@kangax ready to merge for me.
misc fixes nothing special, getLineWidth got more complicated but is necessary.

[kangax]

Great, thanks! Just one minor thing in a comment: "I have to" -> "we have to" or "it has to be" coz it sounds strange otherwise