Make fast clicks still set mouseIsPressed to true

Test Plan:
Wrote the program

    var draw = function() {
        background(255, 255, 255);
        if (mouseIsPressed) {
            background(255, 0, 0);
        }
    };

and verified that tapping on my trackpad makes the screen light up red where it didn't before.

Reviewers: pamela, john

Reviewed By: john

Differential Revision: http://phabricator.khanacademy.org/D9811