CELL: fix stencil twiddling, stencil invert

CELL: fix stencil twiddling, stencil invert

Many stencil tests were failing because of a failure to read the
stencil buffer, due to "twiddling" (or "untwiddling") "an unsupported
texture format".  This is fixed for the case of a stencil/Z S824Z format
(which twiddles just like the 32-bit color formats).

tests/stencilwrap.c was failing on the GL_INVERT test, because
the emitted code for "spe_xori" turned out not to be an actual
"xori" instruction, but rather a "stqd" instruction, because
of a typo in the rtasm code.  This is now fixed, and
tests/stencil_wrap now works.