- Texture: add HasAlpha() to detect whether there are any non--opaque pixels in the texture image. If so, enable alpha-blending during rendering (partly WIP).
- Texture: reworked alpha detection now checks for alpha pixels on the loading thread, causing less rendering disruptions. Also, the SceneRendererClassicGl now updates its table of materials requiring alpha blending after uploading new textures (which could change things).