Invert logic for the single-pass in swScale() functions.

Instead of having a firstTime variable defaulting to 1, have a
warnedAlready defaulting to 0. While this should make no difference in
code speed at runtime, it allows to aggregate the four bytes of that
variable with clip_table in .bss section, rather than issuing a .data
section just for that.

As it is, libswscale require no .data section but .data.rel.ro (that
can be mitigated by prelinking), so the change might actually save one
page of memory at runtime (per process).

Originally committed as revision 27734 to svn://svn.mplayerhq.hu/mplayer/trunk/libswscale