Python GPU: Add reference of PyObject GPU object to the GPU object itself

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

source/blender/python/gpu/gpu_py_framebuffer.h

@@ -28,6 +28,11 @@ extern PyTypeObject BPyGPUFrameBuffer_Type;
 
 typedef struct BPyGPUFrameBuffer {
   PyObject_HEAD struct GPUFrameBuffer *fb;
+
+#if GPU_USE_PY_REFERENCES
+  bool shared_reference;
+#endif
 } BPyGPUFrameBuffer;
 
-PyObject *BPyGPUFrameBuffer_CreatePyObject(struct GPUFrameBuffer *fb) ATTR_NONNULL(1);
+PyObject *BPyGPUFrameBuffer_CreatePyObject(struct GPUFrameBuffer *fb, bool shared_reference)
+    ATTR_NONNULL(1);