Rendering resolution can be different than the native window resolution

- Get rid of the different GlowModes since we found the correct one (DIFFUSE and ADD)
  Get rid of the ui to change the mode

- INtroduce a "RenderResolutionScale" in the application that controls the ratio ofthe resolution between the framebuffers and the native window provided by Qt

- In the paintGL, in the default rendering path:
  the rendering Framebuffers are resized to the window.size * renderResolutionScale.
  The viewport is assigned to the framebuffer size from GlowEffect.prepare() to GlowEffect.render() (and not the original GLCanvas size)
  at the end of GLowEffect render, the final step is to blit the colorbuffer from the rendering into the window's default framebuffer at the original resolution, and that requires one more glViewport

- add the ui in developer/render/ menu "scale resolution" where you can select the scaling level. THis should show in the rendering with ugly aliasing when selecting a scale under 1/2.
Not a particularly big deal, but I'd get rid of the parentheses around _isEmpty here.