[Piglit] PIGLIT_GL_VISUAL_RGBA not what it says on the tin

[Dave Airlie]

So I was debugging texelFetch today with multisample inside softpipe,
when I found in non-fbo mode it was getting alpha values wrong,
becuase

        PIGLIT_GL_VISUAL_RGB            = 0,
        PIGLIT_GL_VISUAL_RGBA           = 0,


I expect someone thought that would get them an alpha visual, whereas
you need to add PIGLIT_GL_VISUAL_ALPHA to get that.

I expect a few other people have fallen into this hole.

Dave.