Default depthCompareOp in DepthStencilState

[franciszhi]

Since #393, reverse depth convention is adopted. For that, depthCompareOp is changed from VK_COMPARE_OP_LESS to VK_COMPARE_OP_GREATER.

In my opinion, VK_COMPARE_OP_GREATER_OR_EQUAL should be used by default as it is the exact reverse of VK_COMPARE_OP_LESS. By doing this, when two objects of same size, same depth are drawn, the visible one should be the the last object commanded to be drawn.

[robertosfield]

The equivalent of VK_COMPARE_OP_LESS with standard increasing depth is VK_COMPARE_OP_GREATER with reverse depth.

One can debate whether _OR_EQUAL is desirable for either case, what is most appropriate all depends upon the specifics of the model you are working with, some applications may find it better, some might find it worse for their models.

What we have is just defaults, you can create objects and if the defaults aren't appropriate you modify them to fit the purpose.