glsl: Define a gl_LastFragData built-in for older GLSL versions.

The EXT_shader_framebuffer_fetch extension defines alternative
language for GLES2 shaders where user-defined fragment outputs are not
allowed.  Instead of using inout user-defined fragment outputs the
shader is expected to read from the gl_LastFragData built-in array.
In addition this allows using the same language on desktop GLSL
versions prior to 4.2 that support the deprecated gl_FragData built-in
in preparation for the MESA_shader_framebuffer_fetch desktop GL
extension.

Both legacy and user-defined inout outputs have a common
representation at the GLSL IR level, so it shouldn't make any
difference for optimization passes and back-ends whether the
application is using gl_LastFragData or user-defined outputs, all
they'll see is a variable dereference of a fragment output at a
certain interface location with the fb_fetch_output bit set to one.

v2: Don't define the built-in variable on GLSL versions for which
    gl_FragData exists but is deprecated. (Ken)

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>

src/compiler/glsl/builtin_variables.cpp

@@ -1134,6 +1134,16 @@ builtin_variable_generator::generate_fs_special_vars()
                  array(vec4_t, state->Const.MaxDrawBuffers), "gl_FragData");
    }
 
+   if (state->has_framebuffer_fetch() && !state->is_version(130, 300)) {
+      ir_variable *const var =
+         add_output(FRAG_RESULT_DATA0,
+                    array(vec4_t, state->Const.MaxDrawBuffers),
+                    "gl_LastFragData");
+      var->data.precision = GLSL_PRECISION_MEDIUM;
+      var->data.read_only = 1;
+      var->data.fb_fetch_output = 1;
+   }
+
    if (state->es_shader && state->language_version == 100 && state->EXT_blend_func_extended_enable) {
       /* We make an assumption here that there will only ever be one dual-source draw buffer
        * In case this assumption is ever proven to be false, make sure to assert here