qt: fix texture blending issue

When the built-in blur function is used, it adjusts the data that is reinterpreted as alpha in the Qt Quick side. (Edges are faded to transparent)

QImage RGB32 format does not report having an alpha channel, but it still has 8 bits per pixel reserved with value of 0xff. The blur function adjusts these bits, and when the texture is generated from the image, the blending of the node material is disabled because the image reports it does not have an alpha channel. However, the fragment shader still respects the alpha value even if blending is disabled.