Fixed OSMesa render window leak.

vtkOSOpenGLRenderWindow was not destroying the OpenGL context when
deleted. Fixed.

Change-Id: I7ad2e9abd7a657ed2c9918d53006cd991098cdbc
Merge topic 'osmesa-leak-fixed' into master

28b5cc99 Fixed OSMesa render window leak.
Merge topic '13484-MathTextPdfExport' into pv-master

3b3130e Replace vtkFreeType(Tools|Utilities) with vtkTextRenderer.
Merge remote-tracking branch 'pvvtk/pv-master' into HEAD

* pvvtk/pv-master:

Change-Id: Ibe1216542f3304233a97b151be4ba940b13c2cec