Avoid shader type error of integer kernal2DSize

[xiasun]

When setting kernel2DSize to an integer in the viewer options, it will be assigned to the GLSL shader as an integer. For example, when setting:

// ...
viewerOptions.kernel2DSize = 1.0;
// ...

The generated shader will look like:

// ...
cov2Dm[0][0] += 1;
cov2Dm[1][1] += 1;
// ...

This raises a type mismatch error during shader compilation.

This fix ensure that at least one decimal is preserved for integer kernel2DSize.