Bugfixes.

 - All GLObjects test to see if opts instances have a parent, before
   doing things with their sync properties.
 - Render.py bug - using fsl/utils/layout functions, passing
   showLabels=True, but then passing no labels. Renderpy usage fixed,
   and layout test added (reverts to showLabels=False if no labels are
   provided)
 - CanvasPanel screenshot was passing all overlays, instead of just
   enabled/saved ones
 - settingspy imports wx inside functions, in case we're running in
   headless mode
 - In my SliceCanvas.genGLObject refactoring, I removed a critical piece
   of functionality - genGLObject(updateRenderTextures=True) was not
   updating render textures.