Fix issues with context lost on Android

This issue could be reproduced on a test-shapes-gles1 simply by switching an app between foreground and background- when resumed libEGL complained about "call to OpenGL ES API with no current context".

Alternative solution would be to trick ```fgPlatformSetWindow()``` into doing it by setting ```fgStructure.CurrentWindow``` to null before calling ```fghPlatformOpenWindowEGL()```, but I think being explicit is cleaner.