This currently sets the base and size of all push constants to the
entire push constant block. The idea is that we'll use the base and size
to eventually optimize the amount we actually push, but for now we don't
do that.
What we had before was kind of a hack where we made certain untrue
assumptions about the incoming data. This new support, while it still
doesn't support indirects properly (that will come), at least pulls the
offsets and strides from SPIR-V like it's supposed to.
