mesa/util: add allow_glsl_relaxed_es driconfig override

This relaxes a number of ES shader restrictions allowing shaders
to follow more desktop GLSL like rules.

This initial implementation relaxes the following:

 - allows linking ES shaders with desktop shaders
 - allows mismatching precision qualifiers
 - always enables standard derivative builtins

These relaxations allow Google Earth VR shaders to compile.

Reviewed-by: Dave Airlie <airlied@redhat.com>

src/compiler/glsl/builtin_functions.cpp

@@ -446,7 +446,8 @@ fs_oes_derivatives(const _mesa_glsl_parse_state *state)
 {
    return state->stage == MESA_SHADER_FRAGMENT &&
           (state->is_version(110, 300) ||
-           state->OES_standard_derivatives_enable);
+           state->OES_standard_derivatives_enable ||
+           state->ctx->Const.AllowGLSLRelaxedES);
 }
 
 static bool