glsl: enable early_fragment_tests implicitly with post_depth_coverage

From ARB_post_depth_coverage:

   "This extension allows the fragment shader to control whether values in
    gl_SampleMaskIn[] reflect the coverage after application of the early
    depth and stencil tests.  This feature can be enabled with the following
    layout qualifier in the fragment shader:

       layout(post_depth_coverage) in;

    Use of this feature implicitly enables early fragment tests."

And a bit later it also adds:

   "early_fragment_tests" requests that fragment tests be performed before
    fragment shader execution, as described in section 15.2.4 "Early Fragment
    Tests" of the OpenGL Specification. If neither this nor post_depth_coverage
    are declared, per-fragment tests will be performed after fragment shader
    execution."

Fixes:
GL45-CTS.post_depth_coverage_tests.PostDepthSampleMask

Reviewed-by: Marek Olšák <marek.olsak@amd.com>

src/compiler/glsl/linker.cpp

@@ -1881,7 +1881,7 @@ link_fs_inout_layout_qualifiers(struct gl_shader_program *prog,
       }
 
       linked_shader->Program->info.fs.early_fragment_tests |=
-         shader->EarlyFragmentTests;
+         shader->EarlyFragmentTests || shader->PostDepthCoverage;
       linked_shader->Program->info.fs.inner_coverage |= shader->InnerCoverage;
       linked_shader->Program->info.fs.post_depth_coverage |=
          shader->PostDepthCoverage;