[MotionVectors] PrevFrame support for user shader code.

[amuTBKT]

Hello

Coming from UE4 background, I think providing user shader logic some context regarding VertexShader invocations can be useful for generating correct motion vectors. In unreal this is handled using PreviousFrameSwitch nodes.

We already handle previous frame data related to Time and Node transforms, but it fails to handle cases where vertex deformation might be generated using some other means. This can be reading texture/physics simulation data passed to material etc.

I would like to propose PREV_FRAME_SWITCH(boolean) input for Vertex shaders.

Changes:

• Added PREV_FRAME_SWITCH vertex shader input. This is only enabled for spatial shaders and Forward+ rendering as other renderers don't support motion vectors.
• PREV_FRAME_SWITCH is defined (or renamed) as "false" for gles3 and mobile shader compiler.
• Added a new VisualShader input node for vertex shader context.

Open to any suggestions/feedback :)

Regards

Amit

Bugsquad edit:

• Closes Add a previous frame context for vertex shaders (for motion vector generation) godot-proposals#7982

[fire]

Are you ok making a proposal? https://github.com/godotengine/godot-proposals/ We can backfill a proposal but it won't be as high quality or exactly what you wanted.

[amuTBKT]

Sure, I have created a new proposal for this.