Initialize OpenEXROutput::m_levelmode() in init().

[jreichel-nvidia]

Avoids conditional jump on uninitialised values in close(). Not sure whether ONE_LEVEL is the best value here ...

Description

Without that fix, valgrind reports a conditional jump on uninitialised values on code like

auto out = OIIO::ImageOutput::create( "exr");

Checklist:

• I have read the contribution guidelines.
• If this is more extensive than a small change to existing code, I
  have previously submitted a Contributor License Agreement
  (individual, and if there is any way my
  employers might think my programming belongs to them, then also
  corporate).
• I have updated the documentation, if applicable.
• I have ensured that the change is tested somewhere in the testsuite
  (adding new test cases if necessary).
• My code follows the prevailing code style of this project.

[lgritz]

LGTM