mesa: fix incorrect uses of FLUSH_CURRENT

FLUSH_CURRENT is used to copy attributes from the vbo module to
Current.Attrib. It flushes vertices too, but that's a side effect,
not the intent.

Reviewed-by: Mathias Fröhlich <mathias.froehlich@web.de>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/merge_requests/3766>

src/mesa/main/context.c

@@ -181,7 +181,7 @@ _mesa_notifySwapBuffers(struct gl_context *ctx)
 {
    if (MESA_VERBOSE & VERBOSE_SWAPBUFFERS)
       _mesa_debug(ctx, "SwapBuffers\n");
-   FLUSH_CURRENT( ctx, 0 );
+   FLUSH_VERTICES(ctx, 0);
    if (ctx->Driver.Flush) {
       ctx->Driver.Flush(ctx);
    }