renderer/metal: need to set proper alpha for fg mode in shader

Fixes #673

src/renderer/shaders/cell.metal

@@ -166,7 +166,7 @@ fragment float4 uber_fragment(
     // We premult the alpha to our whole color since our blend function
     // uses One/OneMinusSourceAlpha to avoid blurry edges.
     // We first premult our given color.
-    float4 premult = float4(in.color.rgb * in.color.a, 1);
+    float4 premult = float4(in.color.rgb * in.color.a, in.color.a);
     // Then premult the texture color
     float a = textureGreyscale.sample(textureSampler, coord).r;
     premult = premult * a;