fix mesh2d_manual example

[awtterpip]

Objective

• Fixes mesh2d_manual example doesn't render correctly #15847

Solution

• Change mesh2d_manual to use the proper vertex format for color.

Testing

• The mesh2d_manual example now works for me, tested on linux

[rparrett]

This comment is no longer needed

[rparrett]

This looks good, but I'm curious about why we changed the vertex attribute format in that migration. I had the impression that we were intentionally using a custom vertex attribute in that example, but I could very easily be mistaken.

[rparrett]

Would like to know if the change away from the custom vertex attribute in #15847 was intentional or desired before approving.

This fixes the issue, but it could also be fixed by switching back to the custom u32 based color attribute, probably. And I think that example may have been doing that intentionally.

[Jondolf]

@rparrett It was half-intentional, but can be reverted. I originally changed it to fix a crash caused by conflicting vertex color formats with default mesh materials (see this) but then fixed it in a different way as per Cart's suggestion. I forgot to change it back and thought it was cleaner like that anyway, since it's the default format that Mesh::ATTRIBUTE_COLOR uses.

My bad though! Didn't anticipate that it would break things or be particularly undesirable (I recall testing it and it worked like normal...)