Fix primitive count calculation for topology conversion (#3763)

Luigi's Mansion 3 performs a non-index quads draw with 6 vertices. It's meant to ignore the last two, but the index pattern's primitive count calculation was rounding up.

No idea why the game does this but this should fix random triangles in the map.

Ryujinx.Graphics.Vulkan/IndexBufferPattern.cs

@@ -38,7 +38,7 @@ namespace Ryujinx.Graphics.Vulkan
 
         public int GetPrimitiveCount(int vertexCount)
         {
-            return Math.Max(0, ((vertexCount - BaseIndex) + IndexStride - 1) / IndexStride);
+            return Math.Max(0, (vertexCount - BaseIndex) / IndexStride);
         }
 
         public int GetConvertedCount(int indexCount)