This adds motion blur support for Grease Pencil.
We follow the same principle form EEVEE and use the existing
`antialiasing_accumulate` (SSAA) function to accumulate the
frames in range of the motion blur over time.
There is a new `motion_blur_steps` setting in the Grease
Pencil render settings to control the accuracy of the motion
blur. This will increase the render time.
Limitations:
* When Grease Pencil is composited into the scene, we only do
it for one the current frame and don't take the motion blur into
account. This will lead to hard edges currently.
* There is no viewport motion blur. This would need an entirely
new technique that can be computed in real-time.
Pull Request: https://projects.blender.org/blender/blender/pulls/139840
f7a0003452