Make ogl_pal a ubyte (*)[256*3] to improve checking
Step powerup frame number carefully

Avoid relying on integer underflow when checking for moving the current
frame number outside the range of frames.
Fix array understep when mapping exit tunnel
Fix memory leak in audio conversion
Annotate mem_malloc/mem_calloc as __attribute__((malloc))
Annotate mem_malloc/calloc/realloc as __attribute__((alloc_size))