Old code dumped the efb, which was no-longer relevant since the
backend gained xfb support.
New code dumps the colour texture which is about to be rendered to
the screen so correctly reflects the bypassXFB option.
- remove unused variables
- reduce the scope where it makes sense
- correct limits (did you know that strcat()'s last parameter does not
include the \0 that is always added?)
- set some free()'d pointers to NULL