Compiling with `-Wfloat-conversion` warns about a few implicit
conversions from `double`/`float` to other numeric types in the `GValue`
transform functions.

These warnings are correct: value transformations can result in loss of
precision. That loss of precision is understood and expected, so add
some explicit casts to squash the warnings.

Signed-off-by: Philip Withnall <pwithnall@gnome.org>

Helps: #3405