draw: finally optimize bool clip mask generation

lp_build_any_true_range is just what we need, though it will only produce
optimal code with sse41 (ptest + set) - but even without it on 64bit x86
the code is still better (1 unpack, 2 movq + or + set), on 32bit x86 it's
going to be roughly the same as before.
While here also make it a "real" 8bit boolean - cuts one instruction but
more importantly similar to ordinary booleans.

Reviewed-by: Jose Fonseca <jfonseca@vmware.com>