EMI: Use half-transparent plane to implement dimScreen (OpenGLS)

- instead of re-calculating the color values when dimming the screen,
  better draw a half-transparent plane after drawing all non-overworld
  characters
- fixes the issue that overworld actors were dimmed and that the
  pause screen is transparent when the inventory is open
Merge pull request #1 from Botje/pr965

EMI: Use half-transparent plane to implement dimScreen (OpenGLS)