Optimise Full Transparency

[viluon]

It could be feasible to keep a single "fully transparent" pixel, that we could compare against in the render methods. It's just an if per draw call, with three comparisons, so the performance impact is probably negligible. The comparison in render methods would avoid (best case scenario) 3 table accesses (caching the pixel contents) and render-time transparency comparison. Worst case scenario, it would avoid 2 comparisons done at render time (3 transparency values, minus the == for the pixel itself). Note that when #11 is resolved, the parent == target check should go next to this pixel comparison.