Fix loading of TKOpenGl

The TKOpenGl library name was previously returned by the
Graphic3d_GraphicDevice::ShrEnvString method.
We had modified it in the past in order to support debug
libraries on Windows.

Our changes did no more apply cleanly on OCCT 6.6.0, and since
OCCT developers claimed that TKOpenGl is no more loaded at
runtime, our changes had been discarded.

Qb noticed that TKOpenGl is still loaded at runtime, so reinstate
our changes, but this time against Graphic3d::InitGraphicDriver.
Merge pull request #385 from tpaviot/review/fix-TKOpengl-loading

Fix loading of TKOpenGl