The buffer passed as an argument to `GPUFrameBuffer.read_color` is used
in the return of the function and therefore, if not used, its refcount is
decremented.
So be sure to increment the refcount of the already existing objects that
will be used in the return of a function.
This commit adds a new API tha allow to replace the bgl API in the exemples on:
https://docs.blender.org/api/current/gpu.html
**Overview (New API):**
```
gpu.state: active_framebuffer_get
GPUFramebuffer: read_color
GPUOffscreen: texture_color
```
Reviewed By: brecht
Differential Revision: https://developer.blender.org/D11031
Instead of creating different python wrappers for the same GPU object,
return the same `PyObject` created earlier.
This also allows for more secure access to existing GPU objects.
Reviewed By: brecht
Differential Revision: https://developer.blender.org/D11044
