Blender Git Loki

Fix T85564: FCurve modifier zero influence on restrict range borders

When using FModifier `Restrict Frame Range`, the resulting influence was
zero being exactly on `Start` / `End` range borders (so borders were
**exclusive**).
This made it impossible to chain FModifers together (forcing the user to
specify values slightly below the desired border in following
FModifiers).
This is now corrected to be **inclusive** on Start / End range borders.

Before
{F10234864}
After
{F10234865}
Testfile
{F10234866}

In the case of touching open borders (so [frame A frame B] followed by
[frame B frame C]) both modifiers are evaluated (in stack order).
If the later modifier has full influence (and is not additive) this simply
means the result is the same as the later modifier's value.
If influences below 1 are used (or modifiers are additive) both modifier's
values are interpolated/added accordingly.

technical notes:
- this was caused by the introduction of FModifier Influence/BlendIn-Out
in rB185663b52b61.
- for comparison, see other occurrences of
`FMODIFIER_FLAG_RANGERESTRICT`.
- the following conditions in `eval_fmodifier_influence` for blend in/
out have been changed accordingly.

Maniphest Tasks: T85564

Differential Revision: https://developer.blender.org/D10401