Transformer: calculate the answer's coords for json

Summary:
Previously, we would pluck the coords from the current
movablepoints and use those in the json. This was all
good and well, except will be very sad when we have a
mode where the points on the graph don't get transformed
by the transformation list (i.e. formal mode 2). Since
we want to be able to grade those, we can recalculate
where the points should be positioned from the start.

This also should slightly reduce the numerical
inaccuracies built up from lots of little transformations
being applied on each tool move.

Test Plan:
Visit test.html, and make sure grading works (empty,
correct, and incorrect). Make sure grading works in the
new formal tool work even when the graph portion is
static.

Reviewers: alex

Reviewed By: alex

CC: alpert

Differential Revision: http://phabricator.khanacademy.org/D5190