Remove 16× MSAA support due to driver bugs and low performance

In the `master` branch, 16× MSAA caused the entire system to freeze
on NVIDIA GPUs. This is likely caused by graphics drivers not actually
implementing 16× MSAA, but combining 8× MSAA with 2× SSAA instead.

On top of that, modern shader complexity makes 16× MSAA very difficult
to use while keeping a good framerate. 8× MSAA is hard enough to use
as it is.

scene/main/viewport.h

@@ -104,7 +104,7 @@ public:
 		MSAA_2X,
 		MSAA_4X,
 		MSAA_8X,
-		MSAA_16X,
+		// 16x MSAA is not supported due to its high cost and driver bugs.
 		MSAA_MAX
 	};