Ensure Motion Vectors are enabled by particles and skeletons when using the Motion Vector debug draw option

[clayjohn]

Fixes issue reported in #92711 (comment)

Particles, MultiMesh, and Skeletons need to know in advance if they should save enough information for Motion Vectors.

Further work

Motion Vectors will still be missing when using a compositor effect since the check currently only happens at draw time here:

godot/servers/rendering/renderer_rd/forward_clustered/render_forward_clustered.cpp

Lines 1645 to 1656 in 292e50e

	bool motion_vectors_required;
	if (using_debug_mvs) {
	motion_vectors_required = true;
	} else if (ce_needs_motion_vectors) {
	motion_vectors_required = true;
	} else if (!is_reflection_probe && using_taa) {
	motion_vectors_required = true;
	} else if (!is_reflection_probe && using_fsr2) {
	motion_vectors_required = true;
	} else {
	motion_vectors_required = false;
	}

We will need to add some sort of registration so if a Compositor effect requests motion vectors all systems are aware that motion vectors are needed

CC @BastiaanOlij

[BastiaanOlij]

Hmmm, it shouldn't be too hard to enhance _viewport_requires_motion_vectors to do so. We simply need to check the compositor associated with the viewport and see if any of the effects have the motion vector flag set.

One thing that makes sense to me here is to implement a helper on the Compositor itself, where a compositor effect informs the compositor it needs motion vectors and the compositor keeps a count. That way we don't need to loop through the compositor effects each time we do this check.

I'll see if I can add something for that and I'll PR it ontop of your PR.

[BastiaanOlij]

This might be enough: #93068

[clayjohn]

This might be enough: #93068

Awesome! Yep that should fix it for CompositorEffects.

Do you mind approving this PR so it is fixed for all cases now?

[Calinou]

Tested locally, it works as expected.

[akien-mga]

Thanks!