Cycles uses pixel buffers to update the display. Due to making things
work the vulkan backend downloaded the GPU allocated pixel buffer to the
CPU, Copied it to a GPU allocated staging buffer and update the display
texture using the staging buffer. Needless to say that a (CPU->)GPU->CPU->GPU
roundtrip is a bottleneck.
This PR fixes this by allowing the pixel buffer to act as a staging
buffer as well.
Viewport and final image rendering performance is now also similar.
| **Render** | **GPU Backend** | **Path tracing** | **Display** |
| ---------- | --------------- | ---------------- | ----------- |
| Viewport | OpenGL | 2.7 | 0.06 |
| Viewport | Vulkan | 2.7 | 0.04 |
| Image | OpenGL | 3.9 | 0.02 |
| Image | Vulkan | 3.9 | 0.02 |
Tested on:
```
Operating system: Linux-6.8.0-49-generic-x86_64-with-glibc2.39 64 Bits, X11 UI
Graphics card: AMD Radeon Pro W7700 (RADV NAVI32) Advanced Micro Devices radv Mesa 24.3.1 - kisak-mesa PPA Vulkan Backend
```
Pull Request: https://projects.blender.org/blender/blender/pulls/133485
2bd4e101a0