Vulkan: make ReadPixels synchronous.
Vulkan: make ReadPixels synchronous.

This makes it so that the PixelBufferDescriptor callback triggers at a
time consistent with other backends.

ReadPixels is still asynchronous in the sense that the callback is
triggered on the main thread. However, it is now guaranteed to trigger
during (or before) flushAndWait(), which is less surprising behavior.

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