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>

clients/simple-egl.c

@@ -133,6 +133,7 @@ init_egl(struct display *display, struct window *window)
 		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
 	};