Don't flip texture coordinates, mirror image instead

If the texture coordinates are flipped, we are going behind the user's
back as they may depend upon the loaded texture coordinates being the
same as in the source file.

Also, flipping the texture coordinates was leading to the w component
of the tangent vector being negated. This in turn led to very broken
behavior when using normal mapping - which depends upon the tangent
vector.

Instead we by default mirror the texture image when loaded via QImage.
We can add API in 5.8 to allow the user to disable this behavior and
allow the user to store the image mirrored on disk which is what should
happen - or even better prepare dds/ktx textures.

Change-Id: Ia63b4e94c5a7cfb6389659fe9be2c6e426f01886
Reviewed-by: Kevin Ottens <kevin.ottens@kdab.com>

src/render/io/objloader.cpp

@@ -241,8 +241,6 @@ bool ObjLoader::load(::QIODevice *ioDev, const QString &subMesh)
                         // Process texture coordinate
                         float s = tokens.floatAt(1);
                         float t = tokens.floatAt(2);
-                        //FlipUVs
-                        t = 1.0f - t;
                         texCoords.append(QVector2D( s, t ));
                     } else {
                         texCoordsOffset++;

src/render/texture/qtexture.cpp

@@ -640,7 +640,7 @@ QTextureImageDataPtr TextureLoadingHelper::loadTextureData(const QUrl &url, bool
             QImage img;
             if (img.load(source)) {
                 textureData = QTextureImageDataPtr::create();
-                textureData->setImage(img);
+                textureData->setImage(img.mirrored());
             }
             break;
         }