This includes 2 functional changes for X11 mailbox behavior:
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It waits till the GPU work is finished before sending the image to X11. Otherwise we can have issues as the GPU implicit fences are only checked on the queued images, which means that otherwise X11 does not correctly keep an idle image on hand.
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Increase the image count to 5. KMS flip queueing + App pipelining of GPU/CPU each eat up one vs. the ideal 3.
With these changes Strange Brigade has the same performance between MAILBOX and IMMEDIATE.