Improve hasVertexAlpha handling

[bghgary]

See https://forum.babylonjs.com/t/babylon-objexport-obj-mesh-serialization-bug/50856.

This is a fix for the first problem indicated in the forum post. The issue is that the vertex alpha flag is inconsistently handled between mesh, geometry, and vertex data causing mesh.flipFaces to create new vertex data with the wrong stride for colors. This change makes the flag more consistent across the board.

[bghgary]

I will also point out that mesh.flipFaces probably shouldn't be extracting every single vertex buffer just to flip the indices/normals. It would be better if mesh.flipFaces only changes the indices and normals without modifying other vertex buffers.

[bjsplat]

Please make sure to label your PR with "bug", "new feature" or "breaking change" label(s). To prevent this PR from going to the changelog marked it with the "skip changelog" label.

[bjsplat]

Please make sure to label your PR with "bug", "new feature" or "breaking change" label(s). To prevent this PR from going to the changelog marked it with the "skip changelog" label.

[bjsplat]

Please make sure to label your PR with "bug", "new feature" or "breaking change" label(s). To prevent this PR from going to the changelog marked it with the "skip changelog" label.

[bghgary]

I split the abstract class code into its own PR: #15160.

[bjsplat]

Please make sure to label your PR with "bug", "new feature" or "breaking change" label(s). To prevent this PR from going to the changelog marked it with the "skip changelog" label.

[bjsplat]

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[bjsplat]

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[bjsplat]

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[bjsplat]

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[bghgary]

After the CI reported some issues and some discussions with @sebavan, we decided that this is not the right way to fix this issue. It means #15160 is not strictly necessary. We may consider reverting it.

The new fix is #15162.