So that the previously anonymous depth/stencil/alpha structures can be
identified in the traces.
This is just syntactic sugar: it does not break source or binary compatibility.
Use the TexFormat->StoreImage() routine for fallbacks. This handles the
case of copying RGBA framebuffer data into an RGBA texture when the base
format is GL_RGB (or GL_LUMINANCE, etc). In that case, we need to set
the texture's alpha=1 and override the framebuffer's alpha.
