This replaces header include guards with `#pragma once`.
A couple of include guards are not removed yet (e.g. `__RNA_TYPES_H__`),
because they are used in other places.
This patch has been generated by P1561 followed by `make format`.
Differential Revision: https://developer.blender.org/D8466
Previously, we had one global `GPU_matrix` stack, so the API was not
thread safe. This patch makes the stack be per `GPUContext`, effectively
making it local per thread (`GPUContext` is located in thread local
storage).
Reviewed By: brecht
Differential Revision: https://developer.blender.org/D5405
