libs/vkd3d: Use negative viewport height to flip along y-axis.

Requires VK_KHR_maintenance1.

demos/gears.vert

@@ -35,6 +35,4 @@ void main()
 
     colour_out.xyz = diffuse_in.xyz * att;
     colour_out.w = 1.0;
-
-    gl_Position.y = -gl_Position.y;
 }

demos/triangle.vert

@@ -8,7 +8,6 @@ layout(location = 0) out vec4 colour_out;
 
 void main(void)
 {
-    gl_Position.xzw = position_in.xzw;
-    gl_Position.y = -position_in.y;
+    gl_Position = position_in;
     colour_out = colour_in;
 }