glsl: Add support for the framebuffer fetch layout(noncoherent) qualifier.

This allows the application to request framebuffer fetch coherency
with per-fragment output granularity.  Coherent framebuffer fetch
outputs (which is the default if no qualifier is present for
compatibility with older versions of the EXT_shader_framebuffer_fetch
extension) will have ir_variable_data::memory_coherent set to true.

Reviewed-by: Plamena Manolova <plamena.manolova@intel.com>

src/compiler/glsl/builtin_variables.cpp

@@ -1194,6 +1194,7 @@ builtin_variable_generator::generate_fs_special_vars()
       var->data.precision = GLSL_PRECISION_MEDIUM;
       var->data.read_only = 1;
       var->data.fb_fetch_output = 1;
+      var->data.memory_coherent = 1;
    }
 
    if (state->es_shader && state->language_version == 100 && state->EXT_blend_func_extended_enable) {