Remove shadow palette from projectiles and WithShadow

[pchote]

This PR implements another incremental step towards supporting RGBA world sprites. The special-case shadow palette for projectiles and aircraft is replaced with a direct color replacement using the new plumbing from #18550. The shadow palette is still used for voxels in TS and torpedos in RA, but can be removed from D2k and TD.

I find it a bit odd that we default to not drawing shadows on projectiles. This must have made sense many years ago, but I don't think it does now. The current PR keeps them off by default, but I think we should flip this. Thoughts?

[pchote]

https://github.com/pchote/OpenRA/tree/d2k-r16 contains a testcase demonstrating shadow support for RGBA sprites (extending my d2k's r16 testcase from #18942).

[reaperrr]

I find it a bit odd that we default to not drawing shadows on projectiles. This must have made sense many years ago, but I don't think it does now. The current PR keeps them off by default, but I think we should flip this. Thoughts?

Apart from projectiles in original RA/TD/D2k usually not having shadows either, part of the reason why at least I haven't bothered adressing this before was that enabling them on cannon bullets for example, just highlights how much the real altitude of them often differs from what the rendered vehicle/gun turret sprites would suggest.
Doesn't mean I'd object to enabling them by default, but it may look at least a little odd in some cases so we might have to fine-tune offsets here and there to make it look better/less odd.

[pchote]

Right, I forgot about normal tank shells. I was thinking mainly about missiles and the slower artillery, which do have shadows.

[pchote]

Rebased and updated.

[pchote]

Rebased and updated.

[pchote]

Rebased.

[reaperrr]

https://github.com/OpenRA/OpenRA/wiki/Changelog-(bleed)/_compare/aaec36c490773b333e76dc6cbfe973da1f42e5a3...93500e541389e05203d563e43ef5d738d63fedca