wayland: Add support for eglSwapInterval

wayland: Add support for eglSwapInterval

The Wayland EGL platform now respects the eglSwapInterval value. The value is
clamped to either 0 or 1 because it is difficult (and probably not useful) to
sync to more than 1 redraw.

The main change is that if the swap interval is 0 then Mesa won't install a
frame callback so that eglSwapBuffers can be executed as often as necessary.
Instead it will do a sync request after the swap buffers. It will block for
sync complete event in get_back_bo instead of the frame callback. The
compositor is likely to send a release event while processing the new buffer
attach and this makes sure we will receive that before deciding whether to
allocate a new buffer.

If there are no buffers available then instead of returning with an error,
get_back_bo will now poll the compositor by repeatedly sending sync requests
every 10ms. This is a last resort and in theory this shouldn't happen because
there should be no reason for the compositor to hold on to more than three
buffers. That means whenever we attach the fourth buffer we should always get
an immediate release event which should come in with the notification for the
first sync request that we are throttled to.

When the compositor is directly scanning out from the application's buffer it
may end up holding on to three buffers. These are the one that is is currently
scanning out from, one that has been given to DRM as the next buffer to flip
to, and one that has been attached and will be given to DRM as soon as the
previous flip completes. When we attach a fourth buffer to the compositor it
should replace that third buffer so we should get a release event immediately
after that. This patch therefore also changes the number of buffer slots to 4
so that we can accomodate that situation.

If DRM eventually gets a way to cancel a pending page flip then the compositors
can be changed to only need to hold on to two buffers and this value can be
put back to 3.

This also moves the vblank configuration defines from platform_x11.c to the
common egl_dri2.h header so they can be shared by both platforms.