Fix "Smoke, electricity, etc. all invisible." with tiles


Fixes http://smf.cataclysmdda.com/index.php?topic=8402.0

I can reproduce this with software rendered on and tiles (non-fullscreen) and several tilesets.
However for some tilesets the acid is visible and smoke is not (Deon's).
For others (MShock24) neither smoke, nor acid nor sparks are visible.
The retro tilsets display all three fields (it seems this tileset does not use transparency as much as the other).

As the transparency is applied through the alpha channel, it seems to be caused by the `display_buffer` not having an alpha channel (only RGB, 8 bit each - `RGB888`). This changes the display buffer to have an alpha channel. It works fine for me and fixes the issue.