Fix morph target motion blur

[greeble-dev]

I didn't update a call to morph_prev_vertex in commit dbb003c (part of #23023), so the prepass shader fails to compile if motion blur is enabled.

Testing

cargo run --example many_morph_targets -- --count 4 --motion-blur
cargo run --example many_morph_targets -- --count 4

[greeble-dev]

I'm adding this issue to the 0.19 milestone - hoping that's reasonable as it's a one-liner and fixes a regression that's new in 0.19.