ui/egl: fix framebuffer reads

ui/egl: fix framebuffer reads

Fix egl_fb_read() to use the (destination) surface size instead of the
(source) framebuffer source for glReadPixels.  Pass the DisplaySurface
instead of the pixeldata pointer to egl_fb_read() to make this possible.

With that in place framebuffer reads work fine even if the surface and
framebuffer sizes don't match, so we can remove the guest-triggerable
asserts in egl_scanout_flush().

Buglink: https://bugzilla.redhat.com//show_bug.cgi?id=1749659
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-id: 20190909073911.24787-1-kraxel@redhat.com