aco: A few minor improvements to NGG GS
This MR adds a few minor improvements to the ACO NGG GS implementation.
It adds a few "sanity check" type assertions and a few minor fixes. Most notably, we now check if the output primitive count is zero, and if so, export a zero-area triangle. This is recommended by AMD developers to make sure to avoid a possible hang in the Navi 1x NGG hardware.