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This adds missing readback bits so that Wayland can be used with
SwiftShader. I encountered this problem when started to work on
enabled tests with wayland backend enabled.

The implementation is quite naive - we create a
shared memory buffer, import a wl_buffer with the fd for that
shm buffer and write pixels that are read with glReadPixels into
the shm buffer.

Also, this patch restores async buffer swap support in
SkiaOutputDeviceGL that was removed in https://crrev.com/c/2025968.
Now, it's Wayland that has the support for async buffer swap, it's
wrong to assume there are no clients that do not support it.

and also running chrome --ozone-platform=wayland --use-gl=swiftshader.

Test: views_unittests to ensure that surface with read back is created
Bug: 1054305
Change-Id: Iff307e1155d0a14965a2bd193a7b9b8d9aceb952
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2064929
Commit-Queue: Maksim Sisov <msisov@igalia.com>
Reviewed-by: Robert Kroeger <rjkroege@chromium.org>
Reviewed-by: Michael Spang <spang@chromium.org>
Cr-Commit-Position: refs/heads/master@{#746758}