I want to do Masters in Computer Graphics and there are very few universities in the US that deeply explore that area namely, Utah, Cornell, Stanford etc.
Are you sure there are very few? This website is a very good resource for seeing which programs are active in an area: CSRankings -- Computer Graphics. It lists many universities with faculty who are actively publishing, including in order: Stanford, MIT, UCSD, CMU, NYU, Cornell, Texas A&M, etc. (Utah is only #19 in this ranking, which is based on number of publications at the top venues).
Since I've a couple of personal research projects already on my Github, I was hoping to email a professor asking for an invitation letter from his university if I appear promising.
This is perfectly appropriate, but it does sound like the kind of email many professors are likely to ignore, especially since you are looking into Masters programs and not PhD. So I would be careful to keep it brief, to the point, and state your qualifications / accomplishments early (and/or link to your CV or webpage which states those things).
However since, this is a Masters, I certainly don't want to get bound
and give my Master's thesis under the same professor as I haven't even
explored all the sub-areas and haven't developed my interests fully.
You would ideally email professors who you would broadly be interested in working for; to not do so is misleading, as you are essentially emailing to open up a connection of mutual interest between you.
However, you should not be worried about being formally bound at this stage. Sending an email is not a formal binding, and as long as you are honest, there should be no hard feelings if your plans change after you join the program or after you are admitted. Research interests inherently change over time, and many people switch to entirely different areas (let alone sub-areas). You can also check with the program to see if students are formally bound to advisors on admission, or later.
should I explicitly mention that I've not made up my mind regarding a specific research area?
I would briefly mention this, yes -- it's along the lines of being honest and not giving a bad impression. Be as specific as you can without being dishonest -- "broadly interested in computer graphics" is fine if that's all you are sure about, but if you know some sub-areas you are interested in (that might align with the professor's), that can help as well.