renderer: fully switch to premultiplied alpha

src/render/shaders/Border.hpp

@@ -111,10 +111,8 @@ void main() {
     if (pixColor[3] == 0.0)
         discard;
 
-    float pixColor3 = pixColor[3];
     pixColor = getColorForCoord(v_texcoord);
-    pixColor[3] *= alpha * pixColor3;
 
-    gl_FragColor = pixColor;
+    gl_FragColor = pixColor * alpha;
 }
 )#";

src/render/shaders/Shadow.hpp

@@ -78,5 +78,10 @@ void main() {
         discard; return;
     }
 
+    // premultiply
+    pixColor[0] *= pixColor[3];
+    pixColor[1] *= pixColor[3];
+    pixColor[2] *= pixColor[3];
+
 	gl_FragColor = pixColor;
 })#";