meson: switch default build type to debugoptimized

meson: switch default build type to debugoptimized

As discussed in this thread:
https://lists.freedesktop.org/archives/mesa-dev/2017-November/175104.html

Cc: Emil Velikov <emil.l.velikov@gmail.com>
Cc: Ilia Mirkin <imirkin@alum.mit.edu>
Cc: Michel Dänzer <michel@daenzer.net>
Cc: Christian Schmidbauer <ch.schmidbauer@gmail.com>
Cc: Eero Tamminen <eero.t.tamminen@intel.com>
Cc: Ernst Sjöstrand <ernstp@gmail.com>
Signed-off-by: Eric Engestrom <eric.engestrom@imgtec.com>
Acked-by: Matt Turner <mattst88@gmail.com>
Acked-by: Andres Rodriguez <andresx7@gmail.com>
Acked-by: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Dylan Baker <dylan@pnwbakers.com>
Reviewed-by: Chad Versace <chadversary@chromium.org>
Tested-by: Chad Versace <chadversary@chromium.org>