This patch refactors the GPU context handing in the GPU compositor.
First, GPU context handling in the GPU compositor constructor was
removed, that's because the constructor does nothing GPU related.
Second, the destructor and the execute methods were unified to use the
global DST context for main thread execution and a dedicated system GPU
context for threaded execution. The former is the case for blocking
rendering as well as background mode, so the blocking due to the global
DST is not an issue, but it makes the code more robust to implementation
errors like #122070.
Pull Request: https://projects.blender.org/blender/blender/pulls/122389
2b1e825545