Fix the reading error when PLY files use non-RGB color orders.

[pengpeng-yu]

Type

• Bug fix (non-breaking change which fixes an issue): Fixes [Open3D WARNING] Read PLY failed: unable to read file: #5994
• New feature (non-breaking change which adds functionality). Resolves #
• Breaking change (fix or feature that would cause existing functionality to not work as expected) Resolves #

Motivation and Context

It seems that open3d.io.read_point_cloud assumes that the color properties end with the blue channel:

Open3D/cpp/open3d/io/file_format/FilePLY.cpp

Lines 88 to 90 in c6d474b

	if (index == 2) { // reading 'blue'
	state_ptr->color_index++;
	}

If a PLY file defines colors in other orders, e.g., GBR, the color_index is incorrectly incremented after reading the blue channel, causing all red values to be placed at the wrong positions.

However, open3d.t.io.read_point_cloud correctly handles this case:

Open3D/cpp/open3d/t/io/file_format/FilePLY.cpp

Lines 52 to 56 in c6d474b

	const int64_t index = attr_state->stride_ * attr_state->current_size_ +
	attr_state->offset_;
	data_ptr[index] = static_cast<T>(ply_get_argument_value(argument));
	++attr_state->current_size_;

Checklist:

• I have run python util/check_style.py --apply to apply Open3D code style
  to my code.
• This PR changes Open3D behavior or adds new functionality.
  • Both C++ (Doxygen) and Python (Sphinx / Google style) documentation is
    updated accordingly.
  • I have added or updated C++ and / or Python unit tests OR included test
    results (e.g. screenshots or numbers) here.
• I will follow up and update the code if CI fails.
• For fork PRs, I have selected Allow edits from maintainers.

Description

I changed the meaning of color_index to track the number of processed channels (instead of the number of points), which allows for correct point indexing regardless of the channel order.

Here is an example using this file :

import open3d as o3d
import numpy as np

pc = o3d.io.read_point_cloud("aaa.ply")   # Current version will produce  RPly: Aborted by user  [Open3D WARNING] Read PLY failed: unable to read file: aaa.ply
pc2 = o3d.t.io.read_point_cloud("aaa.ply")

print( ((pc2.point.colors).numpy() == np.asarray(pc.colors) * 255).all() )

[update-docs]

Thanks for submitting this pull request! The maintainers of this repository would appreciate if you could update the CHANGELOG.md based on your changes.