Resolve clang compilation error: lookup of 'Depth' in member access expr...

<pre>
tests/testRegalPpa.cpp:311:48: error: lookup of 'Depth' in member access expression is ambiguous
  EXPECT_EQ( static_cast<GLclampd>( 1.0 ), ppa.Depth::clear );
                                               ^
src/googletest/include/gtest/gtest.h:1980:33: note: expanded from macro 'EXPECT_EQ'
                      expected, actual)
                                ^
src/googletest/include/gtest/gtest_pred_impl.h:162:40: note: expanded from macro 'EXPECT_PRED_FORMAT2'
  GTEST_PRED_FORMAT2_(pred_format, v1, v2, GTEST_NONFATAL_FAILURE_)
                                       ^
src/googletest/include/gtest/gtest_pred_impl.h:147:43: note: expanded from macro 'GTEST_PRED_FORMAT2_'
  GTEST_ASSERT_(pred_format(#v1, #v2, v1, v2), \
                                          ^
src/googletest/include/gtest/gtest_pred_impl.h:77:52: note: expanded from macro 'GTEST_ASSERT_'
  if (const ::testing::AssertionResult gtest_ar = (expression)) \
                                                   ^
src/regal/RegalState.h:743:8: note: lookup in the object type 'Emu::Ppa' refers here
struct Depth
       ^
/usr/include/X11/Xlib.h:259:3: note: lookup from the current scope refers here
} Depth;
  ^
fatal error: too many errors emitted, stopping now [-ferror-limit=]
</pre>