use pointer to union instead of void pointer.
gcc complained a lot, depending what the pointer originally actually was.
Looks like it's in fact maybe legal to cast for instance uint pointers to
union pointers as long as union contains a uint type, hence use this with some
callers, other just use union util_color in the first place.
This fixes a bug seen when doing a glDrawPixels(GL_STENCIL_INDEX) right
after a glClear(). The check-for-flush test was failing because we
didn't set the dirty_render_cache flag in softpipe_clear(). So we saw
stale data when we mapped the stencil buffer.
