Fixed vignette filter to take into account alpha in the original imag…

…e, improved its performance.

framework/Source/GPUImageVignetteFilter.m

@@ -12,17 +12,10 @@
 
  void main()
  {
-     /*
-     lowp vec3 rgb = texture2D(inputImageTexture, textureCoordinate).rgb;
-     lowp float d = distance(textureCoordinate, vec2(0.5,0.5));
-     rgb *= (1.0 - smoothstep(vignetteStart, vignetteEnd, d));
-     gl_FragColor = vec4(vec3(rgb),1.0);
-      */
-
-     lowp vec3 rgb = texture2D(inputImageTexture, textureCoordinate).rgb;
+     lowp vec4 sourceImageColor = texture2D(inputImageTexture, textureCoordinate);
      lowp float d = distance(textureCoordinate, vec2(vignetteCenter.x, vignetteCenter.y));
      lowp float percent = smoothstep(vignetteStart, vignetteEnd, d);
-     gl_FragColor = vec4(mix(rgb.x, vignetteColor.x, percent), mix(rgb.y, vignetteColor.y, percent), mix(rgb.z, vignetteColor.z, percent), 1.0);
+     gl_FragColor = vec4(mix(sourceImageColor.rgb, vignetteColor, percent), sourceImageColor.a);
  }
 );