st/mesa: set strb->format field in st_render_texture()

This fixes a problem in glReadPixels when reading from an FBO's texture
attachment.  We have a better chance at hitting a fast path for
glReadPixels now.

src/mesa/state_tracker/st_cb_fbo.c

@@ -379,6 +379,8 @@ st_render_texture(GLcontext *ctx,
                                            PIPE_BUFFER_USAGE_GPU_READ |
                                            PIPE_BUFFER_USAGE_GPU_WRITE);
 
+   strb->format = pt->format;
+
    strb->Base.Format = st_pipe_format_to_mesa_format(pt->format);
    strb->Base.DataType = st_format_datatype(pt->format);