About 2 years ago, before I came out, my mum saw this picture of a man who she thought "looked like a woman," and she made some rather transphobic jokes about him. Because of this I worried about what she would think when I came out. To my surprise, she was very accepting, and started doing research on transgender issues, and she no longer makes these kind of jokes. It could be that your coworkers are having a joke simply because they don't know about this subject, they think it's all a bit of fun, they don't know that they're hurting someone in their midst. That doesn't make it right, of course. Totally understandable why you'd feel angry! But it's possible that once they learn better they will buck up their attitudes.
One tactic I used a lot to gauge people's true opinions, is I would find a newspaper article about a trans person (a positive one, of course!) and conversationally ask people what they thought about it. Then I could say things like "I think this person did really well, good on 'em!" and even discuss it to a degree, because people would assume we were talking about the person in the article and no one else.
As to coming out yourself I would recommend researching your company policy and the law in your region if you haven't done so already. Is there someone you can talk to in private? Is there a manager you can have a confidential chat with? Or does your company have a department you can go to for advice? Or an anonymous advice service? I'd recommend going that avenue before coming out to people one on one, both for your safety and your peace of mind.