Remove tile height restriction from `vello_common`, add to `vello_hybrid`

With the changes to alpha mask packing in
https://github.com/linebender/vello/pull/866 the scalar code in
`vello_common` and `vello_cpu` now works for arbitrary tile sizes.

Most tile size configurations I've checked pass the tests, some tile
heights (e.g., 7) cause rounding differences in the rasterized images.

`vello_hybrid`'s shader code assumes a tile height of 4, so a check has
been introducted in that crate.

sparse_strips/vello_common/src/tile.rs

diff --git a/sparse_strips/vello_common/src/tile.rs b/sparse_strips/vello_common/src/tile.rs
index 7a8d944..e6b89fa 100644
--- a/sparse_strips/vello_common/src/tile.rs
+++ b/sparse_strips/vello_common/src/tile.rs
@@ -320,11 +320,6 @@
 }
 
 #[cfg(test)]
-const _: () = if Tile::HEIGHT != 4 {
-    panic!("Can only handle tiles with a height of 4 pixels for now.");
-};
-
-#[cfg(test)]
 mod tests {
     use crate::flatten::{Line, Point};
     use crate::tile::Tiles;

sparse_strips/vello_hybrid/src/lib.rs

diff --git a/sparse_strips/vello_hybrid/src/lib.rs b/sparse_strips/vello_hybrid/src/lib.rs
index 74dbaf1..2e2dac9 100644
--- a/sparse_strips/vello_hybrid/src/lib.rs
+++ b/sparse_strips/vello_hybrid/src/lib.rs
@@ -37,3 +37,8 @@
 pub use scene::Scene;
 pub use util::DimensionConstraints;
 pub use vello_common::pixmap::Pixmap;
+
+#[cfg(test)]
+const _: () = if vello_common::tile::Tile::HEIGHT != 4 {
+    panic!("`vello_hybrid` shaders currently require `Tile::HEIGHT` to be `4`");
+};