simple-egl: fix opaque and 16 bits mode options

In those cases we were writing to the wrong EGL config attribute.

Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
diff --git a/clients/simple-egl.c b/clients/simple-egl.c
index 69d28ea..33e711c 100644
--- a/clients/simple-egl.c
+++ b/clients/simple-egl.c
@@ -133,6 +133,7 @@
 		EGL_RED_SIZE, 1,
 		EGL_GREEN_SIZE, 1,
 		EGL_BLUE_SIZE, 1,
+		EGL_ALPHA_SIZE, 1,
 		EGL_RENDERABLE_TYPE, EGL_OPENGL_ES2_BIT,
 		EGL_NONE
 	};