The spec says that EGL_COLOR_BUFFER_TYPE "must be either
EGL_RGB_BUFFER for an RGB color buffer, or EGL_LUMINANCE_BUFFER for a
luminance color buffer."  However, Config::Config() was setting it to
EGL_NONE, which causes EGLChooseConfigTest to fail on the Ozone
platform (which doesn't override the value).  This sets the proper
value for all platforms (which can still override this default).

Bug: angleproject:3260
Change-Id: Iaf59bd76b11c8d80359c760c10baad3b79c79f9d
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1521310Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Yuly Novikov <ynovikov@chromium.org>
Commit-Queue: Ian Elliott <ianelliott@google.com>