Releases texture memory used by the given bitmap. Useful for
freeing the video memory when a certain panel won't be seen anymore. Video memory
is automatically allocated again when the bitmap is seen the next time, so the
instruction won't do any harm. Example:
bmap - Name or Pointer of Bitmap
The video texture for the bitmap will be deallocated.
bmap_purge(splashmap); // frees the splash screen after game start