This PR attempts to address issue #2333 by adding runtime checks to the Camera class.

The new checks throw IllegalArgumentException and affect 5 setters:

• settViewPort(float, float, float, float)
• setViewPortBottom(float)
• setViewPortLeft(float)
• setViewPortRight(float)
• setViewPortTop(float)

For ease of maintenance, the checks are actually centralized in onViewPortChange(). This assumes onViewPortChange() isn't called often enough for this to cause a measurable performance impact.

In addition to testing for negative widths and heights, I decided to also test for zero-width viewports and zero-height viewports. I found at one instance of division by height, and I can't think of a use case for a zero-width viewport.

Automated tests (using JUnit4) are included in the PR.