renderer/{metal,opengl}: need to premult our cell alpha to have effect

src/renderer/shaders/cell.f.glsl

@@ -39,7 +39,8 @@ void main() {
 
     case MODE_FG:
         a = texture(text, glyph_tex_coords).r;
-        out_FragColor = vec4(color.rgb*a, color.a*a);
+        vec3 premult = color.rgb * color.a;
+        out_FragColor = vec4(premult.rgb*a, a);
         break;
 
     case MODE_FG_COLOR:

src/renderer/shaders/cell.metal

@@ -160,8 +160,12 @@ 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);
+    // Then premult the texture color
     float a = textureGreyscale.sample(textureSampler, coord).r;
-    return in.color * a;
+    premult = premult * a;
+    return premult;
   }
 
   case MODE_FG_COLOR: {