added workaround for a bug in SDL 2.0.12 which prevents transparency

added workaround for a bug in SDL 2.0.12 which prevents transparency

Unfortunately, the current stable release version 2.0.12 of the SDL
library used by R'n'D has a critical bug that causes all transparent
images in R'n'D to be opaque, which hurts the game graphics at various
places.

The cause of this bug is in SDL_ConvertSurface(), which does not copy
the color key anymore.

The bug is documented in the SDL bug tracking system as follows:
https://bugzilla.libsdl.org/show_bug.cgi?id=2979

The bug will be fixed in the next stable release of the SDL library.

This workaround checks if a converted surface should have a color key,
and explicitly sets it if it is missing in the newly created surface.