Switch from face pointers to indices, with the following benefits:
- Halve memory usage required for array (saves 16 MB with 1 million quads).
- Allow better code reuse with index-based utilities.
- Ease future replacement of `SubdivCCGFace` with mesh's face offsets.
- Allow future replacement of array with `Mesh::corner_to_face_map()`
- Potentially lower memory bandwidth required during multires evaluation
- Simplify retrieval of index in `BKE_subdiv_ccg_grid_to_face_index`.
For clarity, `gridfaces` was renamed to `grid_to_face_map`.
Pull Request: https://projects.blender.org/blender/blender/pulls/111078
2228006d5b