intel: Make a renderbuffer wrapping a texture have the same _BaseFormat.

Otherwise, when you asked for the _BaseFormat of an rb wrapping a
GL_RGB texture, you got GL_RGBA because that's what we were storing
the texture data as.

NOTE: This is a candidate for the 8.0 branch.

src/mesa/drivers/dri/intel/intel_fbo.c

@@ -475,7 +475,7 @@ intel_renderbuffer_update_wrapper(struct intel_context *intel,
 
    rb->Format = image->TexFormat;
    rb->InternalFormat = image->InternalFormat;
-   rb->_BaseFormat = _mesa_get_format_base_format(rb->Format);
+   rb->_BaseFormat = image->_BaseFormat;
    rb->Width = mt->level[level].width;
    rb->Height = mt->level[level].height;