Canvas live resize can expose a white border on Windows

On Windows, a Canvas could show a white border or exposed edge while the window was being drag-resized. The grey grid drawing itself was not especially slow in isolation; the visible artefact happened because Avalonia could render an intermediate frame at the new window size before Objo had produced a fresh Canvas Paint command buffer for that size. The host filled the newly-sized Canvas background, then replayed drawing commands generated for the previous size, leaving the newly exposed edge visible until the next paint completed.

The fix stores the size associated with each Canvas command buffer. During an automatic resize repaint, if Avalonia needs an intermediate render at a different size, the previous command buffer is temporarily scaled to cover the new bounds. As soon as the fresh Paint completes, the Canvas returns to exact unscaled drawing. This removes the white border during live resize on Windows.