glsl: do not call link_xfb_stride_layout_qualifiers() for fragment shaders

xfb only applies to the latest stage before the fragment shader, so
there is no need to invoke it in the fragment shader.

Fixes:
KHR-GL45.enhanced_layouts.xfb_stride_of_empty_list
KHR-GL45.enhanced_layouts.xfb_stride_of_empty_list_and_api

v2: do reset only if shaders provide an explicit stride

v3: do not call link_xfb_stride_layout_qualifiers() for fragment shaders
(Timothy)

Reviewed-by: Timothy Arceri <tarceri@itsqueeze.com>
Signed-off-by: Juan A. Suarez Romero <jasuarez@igalia.com>

src/compiler/glsl/linker.cpp

@@ -2251,8 +2251,11 @@ link_intrastage_shaders(void *mem_ctx,
    link_tes_in_layout_qualifiers(prog, gl_prog, shader_list, num_shaders);
    link_gs_inout_layout_qualifiers(prog, gl_prog, shader_list, num_shaders);
    link_cs_input_layout_qualifiers(prog, gl_prog, shader_list, num_shaders);
-   link_xfb_stride_layout_qualifiers(ctx, prog, linked, shader_list,
-                                     num_shaders);
+
+   if (linked->Stage != MESA_SHADER_FRAGMENT)
+      link_xfb_stride_layout_qualifiers(ctx, prog, linked, shader_list,
+                                        num_shaders);
+
    link_bindless_layout_qualifiers(prog, gl_prog, shader_list, num_shaders);
 
    populate_symbol_table(linked);