for OGL renderer W buffer rendering avoid undefined vertex z

see https://github.com/melonDS-emu/melonDS/issues/2017

src/GPU3D_OpenGL_shaders.h

@@ -681,6 +681,7 @@ void main()
 
     vec4 fpos;
     fpos.xy = (((vec2(vPosition.xy) ) * 2.0) / uScreenSize) - 1.0;
+    fpos.z = 0.0;
     fZ = float(vPosition.z << zshift) / 16777216.0;
     fpos.w = float(vPosition.w) / 65536.0f;
     fpos.xy *= fpos.w;