Clear stencil/alpha when switching from 565, minor frag shader simplication

I became worried that shader compilers, in their great wisdom, might not optimally reduce:

fragColor0 = vec4(v.rgb, 0.0);
fragColor0.a = 1.0;

Anyway, not overwriting the 0.0 afterward is probably clearer, in case someone wonders, "why zero?"

-[Unknown]