Fixed Timestep Interpolation (3D)

Adds 3D fixed timestep interpolation to the rendering server.
This does not yet include support for multimeshes or particles.

Co-authored-by: lawnjelly <lawnjelly@gmail.com>

scene/main/viewport.cpp

@@ -5024,6 +5024,13 @@ Viewport::Viewport() {
 #endif // _3D_DISABLED
 
 	set_sdf_oversize(sdf_oversize); // Set to server.
+
+	// Physics interpolation mode for viewports is a special case.
+	// Typically viewports will be housed within Controls,
+	// and Controls default to PHYSICS_INTERPOLATION_MODE_OFF.
+	// Viewports can thus inherit physics interpolation OFF, which is unexpected.
+	// Setting to ON allows each viewport to have a fresh interpolation state.
+	set_physics_interpolation_mode(Node::PHYSICS_INTERPOLATION_MODE_ON);
 }
 
 Viewport::~Viewport() {