Draw explosions before paddle

* The paddle has a solid part, so it should be drawn before the
  explosions
* Also, again disable depth writing for vortex and explosions
Performance optimization for blurring

* Use half-sized (quarter area) FBOs for blurring (quarter-sized (1/16
  area) results in flickering, but could be considered as an option for
  the user)
* Use R11F_G11F_B10F instead of RGBA16F; the precision is enough and we
  don't actually need alpha for the frame buffers (the blend mode
  (GL_ONE, GL_SRC_ALPHA) ignores it)