Use glStencilMask() to partially simulate amask

Alpha won't be correct, though.  Not well tested.

This depends on part of hrydgard/native#226, ddf67c5f72d79a392eea1dbcbd76251352c7c671, but it will conflict with #6679.  All that needs to happen is native updated to tip if that is merged first, or I can juts update the pull.

@daniel229 does this improve anything in Silent Hill?  I don't really know if it will or not.

-[Unknown]