r300g: add fallback for back stencil reference value and masks for r3xx-r4xx

r300g: add fallback for back stencil reference value and masks for r3xx-r4xx

This splits rendering into two passes when front and back stencil
reference value, value mask, or write mask don't match.

The advantages of doing it in the driver instead of in st are:
* SWTCL is executed just once and the resulting vertex buffer is reused
  in the second pass.
* Lower driver overhead due to the fallback being very close to
  the actual draw emission with minimum state change.