Fix crash in WindowThumbnail::updatePaintNode()

Recently, the WindowThumbnail was changed so it properly discards the
texture provider in the releaseResources() function.

But it appears like there's a case where the releaseResources() function
can be called in the windowToTexture() function, which will result in
use after free bugs.

Based on af5a855d, it looks like the
WindowThumbnail item didn't use QQuickItem::releaseResources() to
properly release its graphics resources. It was checking whether the
texture node still has a texture. If there's no texture, it means that
the texture got discarded and it needs to be re-created; otherwise it
is okay to re-use the old texture.

With QQuickItem::releaseResources(), we don't need that workaround
anymore since QtQuick will call the releaseResources() function whenever
it wants the WindowThumbnail to discard graphics resources.

BUG: 444015