Support overall primitive alpha in vecr::renderer

libs/vecr/include/psemek/vecr/renderer.hpp

@@ -13,6 +13,7 @@ namespace psemek::vecr
 	{
 		any mask = {};
 		float blur = 1.f;
+		float alpha = 1.f;
 		any_colorizer colorizer = {};
 		blend_mode blend = vecr::blend;
 	};

libs/vecr/source/renderer.cpp

@@ -81,6 +81,7 @@ namespace psemek::vecr
 
 				auto color = colorize(primitive.colorizer, center, sample);
 				color[3] *= blur;
+				color[3] *= primitive.alpha;
 
 				canvas_(x, y) = gfx::to_coloru8(primitive.blend(gfx::to_colorf(canvas_(x, y)), color));
 			}