glsl: Raise linking error if gl_FragDepth layout is inconsistent

glsl: Raise linking error if gl_FragDepth layout is inconsistent

From the AMD_conservative_depth spec:
   If gl_FragDepth is redeclared in any fragment shader in a program, it
   must be redeclared in all fragment shaders in that program that have
   static assignments to gl_FragDepth. All redeclarations of gl_FragDepth in
   all fragment shaders in a single program must have the same set of
   qualifiers.