RFC: Use half-transparent plane to implement dimScreen in EMI

 EMI: Use half-transparent plane to implement dimScreen

- instead of re-calculating the color values when dimming the screen,
  better draw a half-transparent plane after drawing all non-overworld
  characters
- simplifies the source
- fixes the issue that overworld actors were dimmed

Status for the various graphic drivers:
- OpenGL: works
- TinyGL: implemented, but does not work yet (TinyGL does not fully support blending for non-texture objects)
- OpenGLS: not implemented