Reuse the old preview buffer/window such that display-buffer behaves

[bram85]

Before, the buffer is killed and then recreated. This causes wrong behavior for display-buffer when display-buffer-reuse-window and display-buffer-same-window is used: the first one fails so the same window is used where the diagram source is situated.

Instead, reuse the buffer and its window, so don't kill the buffer, erase it instead.