Camera Viewport Dimensions not Checked

[codex128]

If the user accidentaly flips the camera's viewport so that left > right or bottom > top, a negative width/height will be sent to glViewport and glScissor which will result in a quiet OpenGL error and flickering issues. It is super easy to mix up the camera's viewport parameters, so I suggest adding a check to ensure that the camera's viewport width and height remain positive.