[GL3+] Fix issue with EGL_HEADLESS and NVIDIA cards (Linux)

If NVIDIA drivers were installed, the first NV card would always take
preference even if another card had been chosen (i.e. even SW Mesa
selection was being overriden)

This also fixes GLX failing to launch if all the following conditions
were true:
- Non NV card is primary GLX / X Server
- Ogre was build with both GLX and EGL support
- GLX Interface was chosen

RenderSystems/GL3Plus/src/windowing/EGL/PBuffer/OgreEglPBufferSupport.cpp

@@ -407,6 +407,7 @@ namespace Ogre
 
         if( deviceData.eglCtx )
         {
+            eglMakeCurrent( deviceData.eglDisplay, 0, 0, 0 );
             eglDestroyContext( deviceData.eglDisplay, deviceData.eglCtx );
             deviceData.eglCtx = 0;
         }