[client] egl: always render desktop texture as opaque

We ask for 32-bit colour buffer when creating the EGL context. On Wayland,
this sometimes give contexts with alpha channels, resulting in unwanted
transparency. So we clear the alpha channel in the desktop shader.

We also switch to using constant alpha for blending the splash, which
avoids more alpha issues.

client/renderers/EGL/shader/desktop_rgb.frag

@@ -84,4 +84,6 @@ void main()
     color *= 1.0 + lumi;
     color *= nvGain;
   }
+
+  color.a = 1.0;
 }