NBS fullscreen overlay benchmark performance is much lower than BufferQueue

NBS fullscreen benchmark performance is much lower than BufferQueue.

$ sudo bin/mir_proving_server --nbuffers=0 &
$ sudo env MIR_CLIENT_PERF_REPORT=log glmark2-mir --fullscreen -b texture
=======================================================
    glmark2 2014.03+git20150611.fa71af2d
=======================================================
    OpenGL Information
    GL_VENDOR: Intel Open Source Technology Center
    GL_RENDERER: Mesa DRI Intel(R) Haswell Desktop
    GL_VERSION: 3.0 Mesa 11.1.2
=======================================================
[texture] <default>:[2016-03-16 16:47:48.337501] perf: : 1270.00 FPS, render time 0.50ms, buffer lag 1.84ms (3 buffers)
[2016-03-16 16:47:49.337541] perf: : 1284.00 FPS, render time 0.44ms, buffer lag 1.88ms (3 buffers)
[2016-03-16 16:47:50.338014] perf: : 1311.00 FPS, render time 0.45ms, buffer lag 1.83ms (3 buffers)
[2016-03-16 16:47:51.338506] perf: : 1299.00 FPS, render time 0.45ms, buffer lag 1.85ms (3 buffers)
[2016-03-16 16:47:52.338721] perf: : 1309.00 FPS, render time 0.45ms, buffer lag 1.84ms (3 buffers)
[2016-03-16 16:47:53.338970] perf: : 1296.00 FPS, render time 0.44ms, buffer lag 1.86ms (3 buffers)
[2016-03-16 16:47:54.339363] perf: : 1300.00 FPS, render time 0.45ms, buffer lag 1.85ms (3 buffers)
[2016-03-16 16:47:55.339693] perf: : 1299.00 FPS, render time 0.44ms, buffer lag 1.86ms (3 buffers)
[2016-03-16 16:47:56.339954] perf: : 1300.00 FPS, render time 0.44ms, buffer lag 1.85ms (3 buffers)
[2016-03-16 16:47:57.340275] perf: : 1306.00 FPS, render time 0.44ms, buffer lag 1.85ms (3 buffers)
 FPS: 1300 FrameTime: 0.769 ms
=======================================================
                                  glmark2 Score: 1300
=======================================================

$ sudo bin/mir_proving_server &
$ sudo env MIR_CLIENT_PERF_REPORT=log glmark2-mir --fullscreen -b texture
=======================================================
    glmark2 2014.03+git20150611.fa71af2d
=======================================================
    OpenGL Information
    GL_VENDOR: Intel Open Source Technology Center
    GL_RENDERER: Mesa DRI Intel(R) Haswell Desktop
    GL_VERSION: 3.0 Mesa 11.1.2
=======================================================
[texture] <default>:[2016-03-16 16:48:06.510089] perf: : 1795.00 FPS, render time 14238.16ms, buffer lag 1.68ms (4 buffers)
[2016-03-16 16:48:07.510343] perf: : 1884.00 FPS, render time 0.44ms, buffer lag 1.67ms (4 buffers)
[2016-03-16 16:48:08.510718] perf: : 1898.00 FPS, render time 0.44ms, buffer lag 1.66ms (4 buffers)
[2016-03-16 16:48:09.511066] perf: : 1894.00 FPS, render time 0.44ms, buffer lag 1.66ms (4 buffers)
[2016-03-16 16:48:10.511537] perf: : 1877.00 FPS, render time 0.44ms, buffer lag 1.68ms (4 buffers)
[2016-03-16 16:48:11.511716] perf: : 1844.00 FPS, render time 0.44ms, buffer lag 1.72ms (4 buffers)
[2016-03-16 16:48:12.511879] perf: : 1862.00 FPS, render time 0.44ms, buffer lag 1.69ms (4 buffers)
[2016-03-16 16:48:13.512307] perf: : 1842.00 FPS, render time 0.45ms, buffer lag 1.71ms (4 buffers)
[2016-03-16 16:48:14.512818] perf: : 1847.00 FPS, render time 0.44ms, buffer lag 1.71ms (4 buffers)
[2016-03-16 16:48:15.513197] perf: : 1854.00 FPS, render time 0.45ms, buffer lag 1.70ms (4 buffers)
 FPS: 1863 FrameTime: 0.537 ms
=======================================================
                                  glmark2 Score: 1863
=======================================================