Fix image mapping in IE

[BLD-413](https://edx-wiki.atlassian.net/browse/BLD-413)

In the IE `pos_x` and `pos_y` are floats, but should be integers. Float numbers are not supported by [regExp](https://github.com/edx/edx-platform/blob/master/common/lib/capa/capa/responsetypes.py#L2095).
Additional rounding is added.

@valera-rozuvan , @auraz please review.