Scaling of cdata at the axis level

Basically, use `point meta min/max` at the axis level, following the comment:

    % TODO Use caxis not only for color bars.

And https://github.com/matlab2tikz/matlab2tikz/pull/386#issuecomment-64223356 in `rewritePatch`.
Here is the problematic line that leads to the misbehavior of testcase 96. You introduced that check with this PR.