[vulkan] Point list rendering improvements

Implement register and vertex-shader-written point size. Fix point size
computation in geometry shader (convert pixel size to NDC scale using
viewport size). Fix point sprite coordinate generation.

src/xenia/gpu/vulkan/pipeline_cache.h

@@ -281,6 +281,7 @@ class PipelineCache {
     uint32_t sq_context_misc;
     uint32_t rb_colorcontrol;
     float rb_alpha_ref;
+    uint32_t pa_su_point_size;
 
     SetDynamicStateRegisters() { Reset(); }
     void Reset() { std::memset(this, 0, sizeof(*this)); }