Fix float/int cast for vector2 and vector3 GUI elements

[brentyi]

Fixes a regression on #57.

Concretely:

• 0 and 0.0 are the same in Javascript.
• As a result, we were sometimes getting 0 when we actually wanted 0.0.
• This is usually fine because of Python's numeric tower, but can cause some edge cases when combined with numpy.