Fixed a premultiplied alpha shader issue that lead to artifacting when brightness was adjusted.

resources/shaders/glsl/core.glsl

@@ -73,6 +73,7 @@ void main()
 
     // Brightness.
     if (brightness != 0.0) {
+        sampledColor.rgb /= sampledColor.a;
         sampledColor.rgb += 0.3 * brightness;
         sampledColor.rgb *= sampledColor.a;
     }

resources/shaders/glsl/scanlines.glsl

@@ -158,6 +158,7 @@ void main()
 
     // Brightness.
     if (brightness != 0.0) {
+        colorTemp.rgb /= colorTemp.a;
         colorTemp.rgb += 0.3 * brightness;
         colorTemp.rgb *= colorTemp.a;
     }