shader: only write all cbufs after use ones.

Some shaders seem to set this flag when dual-src is enabled,
we don't want to trash the cbuf1 write in that case.

Reviewed-by: Jakob Bornecrantz <jakob@collabora.com>
Signed-off-by: Jakob Bornecrantz <jakob@collabora.com>

src/vrend_shader.c

@@ -994,7 +994,7 @@ static int emit_cbuf_writes(struct dump_ctx *ctx)
    int i;
    char *sret;
 
-   for (i = 1; i < 8; i++) {
+   for (i = ctx->num_outputs; i < 8; i++) {
       snprintf(buf, 255, "fsout_c%d = fsout_c0;\n", i);
       sret = add_str_to_glsl_main(ctx, buf);
       if (!sret)