Fix "no depth test" and render_priority sorting

[clayjohn]

supercedes #29113

Fixes #29108 and fixes #27739.

In GLES3, instead of rendering the sky earlier, render the "no depth test" objects as part of alpha pass.

In GLES2, move the sky rendering after opaque pass, render "no depth test" objects as part of alpha pass. And use priority in alpha pass instead of sort key (which is only used in GLES3).

This was the fix that reduz prefers, as discussed on IRC.

[akien-mga]

Thanks!

[akien-mga]

Cherry-picked for 3.1.2.