MORTEVIELLE: Fix access to free'ed memory

Two buffers were allocated and then free'ed in initialise(), which caused
memory issues when accessed later on throughout the game. They are now
free'ed from the MortevielleEngine destructor.
MORTEVIELLE: Fix memory leak with the _screenSurface

The _screenSurface was not free'ed after being created. This is now
done at the end of run() (the Surface is created at the start of run()
when calling initialise()). Also moved a few free() from the destructor
to the end of run() in case run() can be called more than once on the
same engine.
Merge pull request #1 from criezy/mortevielle

Mortevielle