simple-egl opacity fixes and a couple of warning fixes

This is mostly about making simple-egl do the right thing (opaque mode should generate an XRGB8888 buffer which is also actually opaque, not an ARGB8888 buffer which is translucent?), for values of 'right' which include avoiding 30bpc configs.

There are also two warning fixes thrown in with that, because they were annoying me.