nir: add a lower_mediump_io callback into options

This will be called by the GLSL linker before nir_opt_varyings.

Reviewed-by: Alyssa Rosenzweig <alyssa@rosenzweig.io>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/26918>

src/compiler/nir/nir.h

@@ -4063,6 +4063,11 @@ typedef struct nir_shader_compiler_options {
 
    /** Options determining lowering and behavior of inputs and outputs. */
    nir_io_options io_options;
+
+   /** Driver callback where drivers can define how to lower mediump.
+    *  Used by nir_lower_io_passes.
+    */
+   void (*lower_mediump_io)(struct nir_shader *nir);
 } nir_shader_compiler_options;
 
 typedef struct nir_shader {

src/compiler/nir/nir_lower_io.c

@@ -3259,5 +3259,8 @@ nir_lower_io_passes(nir_shader *nir, bool renumber_vs_inputs)
    if (nir->xfb_info)
       NIR_PASS_V(nir, nir_io_add_intrinsic_xfb_info);
 
+   if (nir->options->lower_mediump_io)
+      nir->options->lower_mediump_io(nir);
+
    nir->info.io_lowered = true;
 }