Merge duplicated motion blur settings between Cycles and EEVEE,
and move them to `RenderData`/`scene.render`:
* `scene.cycles.motion_blur_position` -> `scene.render.motion_blur_position`
* `scene.eevee.use_motion_blur` -> `scene.render.user_motion_blur`
* `scene.eevee.motion_blur_position` -> `scene.render.motion_blur_position`
* `scene.eevee.motion_blur_shutter` -> `scene.render.motion_blur_shutter`
On the C/C++ side, this also renames `RenderData::blurfac` to
`RenderData::motion_blur_shutter`.
Pull Request: https://projects.blender.org/blender/blender/pulls/117913
74b8f99b43