Weapon animation looks choppy

See https://github.com/OpenMW/openmw/blob/master/apps/openmw/mwmechanics/character.cpp#L1398

Weapon animations are comprised of 'sections' like these:

• 'start' to 'min attack'
• 'min attack' to 'max attack'
• 'max attack' to 'min hit'
• 'min hit' to 'hit' etc.

The character controller plays all of these sections separately. It'll start the first section, checks if it's finished, if so starts the next one, and so on. When a section finishes, the remainder of the frametime effectively gets discarded, resulting in a momentary 'pause', a choppy looking animation and the overall animation takes slightly longer to complete than it should. This unpleasant effect is more visible the lower the framerate is.

Assuming these sections follow each other in the NIF file (I'd imagine they do) we should be able to play them as one 'continuous' animation group, though the min/max attack will still need special handling (it has to skip to the correct point depending on the AttackStrength).

(RM-4127 from redmine: created on 2017-09-25 by scrawl ., )