... users would be able to bypass the range checks and build a
defined Rust value which would be interpreted as `None` in C code.
Added format module examples for formatted values constructions.
- The `GST_FORMAT_PERCENT_SCALE` was not used to compute the value
to display.
- Added `Display` examples in the format module documentation.
- Simplified `glib_newtype_display` macro.
The `Signed` version for `GenericFormattedValue` was implemented as
`Signed<GenericFormattedValue>`, which failed to represent properly
the `None` variants when applicable and could represent inconsistent
`Signed` variant combined with `GenericFormattedValue` formats which
are internaly represented as signed integers.