EEVEE: Motion Blur: Add accumulation motion blur for better precision

This revisit the render pipeline to support time slicing for better motion
blur.

We support accumulation with or without the Post-process motion blur.

If using the post-process, we reuse last step next motion data to avoid
another scene reevaluation.

This also adds support for hair motion blur which is handled in a similar
way as mesh motion blur.

The total number of samples is distributed evenly accross all timesteps to
avoid sampling weighting issues. For this reason, the sample count is
(internally) rounded up to the next multiple of the step count.

Only FX Motion BLur: {F8632258}

FX Motion Blur + 4 time steps: {F8632260}

FX Motion Blur + 32 time steps: {F8632261}

Reviewed By: jbakker

Differential Revision: https://developer.blender.org/D8079

release/scripts/startup/bl_ui/properties_render.py

@@ -176,6 +176,7 @@ class RENDER_PT_eevee_motion_blur(RenderButtonsPanel, Panel):
         col.prop(props, "motion_blur_shutter")
         col.prop(props, "motion_blur_depth_scale")
         col.prop(props, "motion_blur_max")
+        col.prop(props, "motion_blur_steps", text="Steps")
 
 
 class RENDER_PT_eevee_depth_of_field(RenderButtonsPanel, Panel):