sparse_strips: Make tile size configurable, Variant 1

[LaurenzV]

We've on various occasions discussed that we want to experiment with different tile sizes. This is especially relevant for vello_hybrid, but also at least worth trying for vello_cpu.

Before we can do that, the first step is to make tile sizes more configurable. Right now, the width and height of 4 is hardcoded as an associated constant in vello_common. This PR changes it so that they become const generics instead, such that different crates can choose their own sizes. In order to keep the diff small for now, we simply define type aliases in vello_hybrid and vello_cpu that link to the existing 4x4 tile. The main algorithms (tiling, strip rendering) for now also still assume 4x4, changing this will be a follow-up PR.

[LaurenzV]

Do you guys think it makes sense to turn those into a generic instead with a sealed trait with variants for 4x4, 8x8 and 16x16 provided such that 1) Clients cannot accidentally create other tile sizes and 2) Later on when we will have to wire the generic through Tiles and StripGenerator, we only have 1 generic parameter instead of 2? I still think it makes sense to keep the distinction between Tile::WIDTH and Tile::HEIGHT internally, but not sure how much sense it makes to expose non-uniform tile sizes to the outside.

[LaurenzV]

I opened a second version here #1656. Curious to hear your thoughts @grebmeg @tomcur @taj-p.

I think I prefer the variant in this PR.