`distribute_natural_allocation` expects an input >= 0 of type `gint`. In
`get_preferred_size_for_opposite_orientation` it is used with an unchecked
variable `size` of type `gfloat`, which in case it is `Infinity`, gets
passed on in the macro `MAX (0, size)`. `Infinity` becomes `G_MININT`
when implicitly casted to `gint` in `distribute_natural_allocation`,
triggering the assertion `extra_space >= 0`.

The resulting warning in the log is counter intuitive and not very
helpful.

Use `float` in `distribute_natural_allocation` instead of `gint` and
assert on denormal values so we can more easily identify bugs.

Additionally change some types while at it and add a even more
expressive warning referencing the actor at one point.

GNOME/mutter!375