render/vulkan: use stage buffers for read_pixels

No need to roll our own cache machinery here, stage buffers
already do it (and more) for us. Should improve performance
when multiple screens with a different resolution are captured.

Closes: wlroots/wlroots#3525
3 jobs for vk-read-pixels-stage-buf in 1 minute and 55 seconds (queued for 10 seconds)
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