Use a shorter/simpler license convention, stops the header taking so
much space.
Follow the SPDX license specification: https://spdx.org/licenses
- C/C++/objc/objc++
- Python
- Shell Scripts
- CMake, GNUmakefile
While most of the source tree has been included
- `./extern/` was left out.
- `./intern/cycles` & `./intern/atomic` are also excluded because they
use different header conventions.
doc/license/SPDX-license-identifiers.txt has been added to list SPDX all
used identifiers.
See P2788 for the script that automated these edits.
Reviewed By: brecht, mont29, sergey
Ref D14069
Using `FunctionRef` is better than using `std::function`, templates and c function
pointers in some cases. The trade offs are explained in more detail in code documentation.
The following are some of the main benefits of using `FunctionRef`:
* It is convenient to use with all kinds of callables.
* It is cheaper to construct, copy and (possibly) call compared to `std::function`.
* Functions taking a `FunctionRef` as parameter don't need to be declared
in header files (as is necessary when using templates usually).
Differential Revision: https://developer.blender.org/D10476
