Fix failure to optimize away alpha channel on some GLTF upload paths (#606)

Co-authored-by: RunitaiLinden <davep@lindenlab.com>

indra/newview/lltinygltfhelper.cpp

@@ -178,6 +178,7 @@ LLImageRaw * LLTinyGLTFHelper::getTexture(const std::string & folder, const tiny
     {
         rawImage = new LLImageRaw(&image->image[0], image->width, image->height, image->component);
         rawImage->verticalFlip();
+        rawImage->optimizeAwayAlpha();
     }
 
     return rawImage;