Replace copyright and license in all source and header files

[garyo]

we should probably change the copyright in the header files now that OpenFX is officially an Academy Software Foundation project. I'm happy to do that (using a script). We are using the BSD 3-Clause License.

I assume we should use a modern SPDX-License-Identifier comment block; perhaps something like this:

// Copyright (c) Contributors to the OpenFX project.
// SPDX-License-Identifier: BSD-3-Clause
// https://github.com/AcademySoftwareFoundation/OpenFx

Is that sufficient or should we add the license text itself, a copyright year, or anything else? Is the copyright attribution correct? (I got it from OpenShadingLanguage and OpenColorIO's header is similar.)

By the way, I notice a few // style comments in the headers although we mostly avoid them. I presume we are OK with this now, since // comments were added to ANSI C in 1999. Let me know if you have any problem with that.

[revisionfx]

New file can bear new (c)

The theory from our transition research for this, as it would have otherwise required we get a signature from all who may have contributed (and their successors for those who are not alive)
Old files we edit should have 2 lines, (c) OpenFX 2003-2022
(c) new one 2022-23

[jmertic]

From our perspective, only the first two lines are required...

// Copyright Contributors to the OpenFX project.
// SPDX-License-Identifier: BSD-3-Clause

Note that I removed the (c) as it's somewhat redundant. Also, adding the year really doesn't add any value, and puts an additional burden on the project to change the year on each file every year.

[garyo]

Now being implemented in #110