This technically belongs to HDSkins

src/client/resources/minelp.mixin.json

@@ -9,7 +9,6 @@
         "MixinGlStateManager",
         "MixinItemRenderer",
         "MixinRenderItem",
-        "MixinRenderManager",
-        "MixinThreadDownloadImageData"
+        "MixinRenderManager"
     ]
 }

src/hdskins/java/com/minelittlepony/hdskins/mixin/MixinThreadDownloadImageData.java

@@ -1,4 +1,4 @@
-package com.minelittlepony.client.mixin;
+package com.minelittlepony.hdskins.mixin;
 
 import net.minecraft.client.renderer.ThreadDownloadImageData;
 import org.spongepowered.asm.mixin.Mixin;

src/hdskins/resources/hdskins.mixin.json

@@ -8,6 +8,7 @@
         "MixinGuiMainMenu",
         "MixinImageBufferDownload",
         "MixinNetworkPlayerInfo",
-        "MixinSkullRenderer"
+        "MixinSkullRenderer",
+		"MixinThreadDownloadImageData"
     ]
 }