Stack corruption in TColorGradient on mac beta

[hageboeck]

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Description

The tutorials intermittently crash as follows:

 1998/3684 Test #1158: tutorial-visualisation-gl-grad2 ...................................................................***Failed    0.54 sec
  Processing /Users/sftnight/ROOT-CI/src/tutorials/visualisation/gl/grad2.C...
  CMake Error at /Users/sftnight/ROOT-CI/src/cmake/modules/RootTestDriver.cmake:253 (message):
    error code: Subprocess aborted

Attaching lldb, this is due to a corruption of the stack canary when -O2 is used. This might be a compiler or library bug, so we'll need to observe whether XCode updates fix it.

For now, the expected failure will be masked, and if the test starts to pass, it will be signalled.

[dpiparo]

I removed the label "bug" since it could be a bug in the compiler/library on beta and not ROOT.