mesa/st: add a gl_program struct flag to skip psiz exports for xfb

if this output did not exist in the original shader,
then it must not be exported in xfb

Reviewed-by: Ilia Mirkin <imirkin@alum.mit.edu>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/15228>

src/mesa/main/shader_types.h

@@ -531,6 +531,9 @@ struct gl_program
    /** Is this program written to on disk shader cache */
    bool program_written_to_cache;
 
+   /** whether to skip VARYING_SLOT_PSIZ in st_translate_stream_output_info() */
+   bool skip_pointsize_xfb;
+
    /** A bitfield indicating which vertex shader inputs consume two slots
     *
     * This is used for mapping from single-slot input locations in the GL API

src/mesa/state_tracker/st_glsl_to_nir.cpp

@@ -304,6 +304,8 @@ st_nir_preprocess(struct st_context *st, struct gl_program *prog,
          st->ctx->SoftFP64 = glsl_float64_funcs_to_nir(st->ctx, options);
    }
 
+   prog->skip_pointsize_xfb = !(nir->info.outputs_written & VARYING_BIT_PSIZ);
+
    /* ES has strict SSO validation rules for shader IO matching so we can't
     * remove dead IO until the resource list has been built. Here we skip
     * removing them until later. This will potentially make the IO lowering

src/mesa/state_tracker/st_program.c

@@ -472,6 +472,12 @@ st_translate_stream_output_info(struct gl_program *prog)
    memset(output_mapping, 0, sizeof(output_mapping));
 
    for (unsigned attr = 0; attr < VARYING_SLOT_MAX; attr++) {
+      /* this output was added by mesa/st and should not be tracked for xfb:
+       * drivers must check var->data.explicit_location to find the original output
+       * and only emit that one for xfb
+       */
+      if (prog->skip_pointsize_xfb && attr == VARYING_SLOT_PSIZ)
+         continue;
       if (prog->info.outputs_written & BITFIELD64_BIT(attr))
          output_mapping[attr] = num_outputs++;
    }