Effects: CC Smear and CC Tiler on text and shape layers reach past the content's box (#659)

A text or shape layer's effect bounds are comp-sized (as in After Effects),
but its buffer surrounds only its content, and each effect grows the buffer
itself when it needs room. CC Smear and CC Tiler never did, so a smear was
cut off where the glyphs end and tiles filled only the text's box.

CC Tiler now covers the whole layer rectangle; CC Smear grows by how far it
moves pixels (Reach x From->To), at most to the layer bounds, so footage
layers, whose buffer is their bounds already, are unchanged. CPU and GPU
alike.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01QiAJeFttZp83Fh7YuzAR5f

crates/effects/src/util.rs

@@ -433,6 +433,17 @@ pub fn layer_rect_at(ctx: &crate::EffectCtx, offset: [f64; 2], scale: f64) -> (f
     (offset[0], offset[1], ctx.layer_size[0] * scale, ctx.layer_size[1] * scale)
 }
 
+/// Transparent pixels to add on every side ([`crate::Buf::pad`]) of a buffer `size` pixels big,
+/// with `offset` and `scale`, so it covers the whole layer rectangle (at most 4096). A text or
+/// shape layer's buffer only surrounds its content, but its layer bounds are comp-sized (see
+/// the renderer's `effect_bounds`): an effect that fills the layer, such as CC Tiler, fills
+/// those as in After Effects (#659).
+pub fn layer_cover_pad(ctx: &crate::EffectCtx, size: [u32; 2], offset: [f64; 2], scale: f64) -> u32 {
+    let (x0, y0, w, h) = layer_rect_at(ctx, offset, scale);
+    let need = [-x0, -y0, x0 + w - f64::from(size[0]), y0 + h - f64::from(size[1])].into_iter().fold(0.0, f64::max);
+    need.ceil().min(4096.0) as u32
+}
+
 /// Resample another layer (a layer parameter) into `b`'s pixel grid. With `stretch` the other
 /// layer is scaled to this layer's size ("Stretch to Fit"); otherwise it is centred.
 pub fn fit_layer(ctx: &crate::EffectCtx, b: &crate::Buf, other: &crate::LayerPixels, stretch: bool) -> Image {