Fix rounding errors for Acroform elements

[rbuetzer]

This pull request fixes the following issue:

The current calculateCoordinates method rounds the coordinates down to the next integer. This leads to the problem, that form elements are not perfectly aligned with other elements.

Example code to reproduce:

    var doc = new jsPDF();

    var radioGroup = new RadioButton();
    radioGroup.V = "/Test";
    radioGroup.Subtype = "Form";
    doc.addField(radioGroup);

    var radioButton1 = radioGroup.createOption("Test");
    radioButton1.Rect = [50, 50, 4, 4];
    radioButton1.AS = "/Test";

    doc.rect(50, 50, 4, 4);

    doc.save('Test.pdf');

Expected behaviour: The radio button is aligned with the rectangle
Actual behaviour: The radio button overlaps the rectangle at the right and at the bottom