Fix T94202: GPUFrameBuffer: wrong refcount in the buffer passed for read_color and read_depth

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.

source/blender/python/gpu/gpu_py_framebuffer.c

@@ -530,6 +530,7 @@ static PyObject *pygpu_framebuffer_read_color(BPyGPUFrameBuffer *self,
       PyErr_SetString(PyExc_BufferError, "the buffer size is smaller than expected");
       return NULL;
     }
+    Py_INCREF(py_buffer);
   }
   else {
     py_buffer = BPyGPU_Buffer_CreatePyObject(
@@ -590,6 +591,7 @@ static PyObject *pygpu_framebuffer_read_depth(BPyGPUFrameBuffer *self,
       PyErr_SetString(PyExc_BufferError, "the buffer size is smaller than expected");
       return NULL;
     }
+    Py_INCREF(py_buffer);
   }
   else {
     py_buffer = BPyGPU_Buffer_CreatePyObject(GPU_DATA_FLOAT, (Py_ssize_t[]){h, w}, 2, NULL);