Few fixes and improvements.

[patlefort]

Mostly some code improvements that remove warnings. I ran changed files through clang-format, is it fine?

Bugs:

• Fix missing break in DoCDROMAccess.
• Fix missing include for va_list and change to std::va_list.
• Fix missing initializer for VkClearDepthStencilValue. Not sure what value it should have or if it even matter as I'm not very familiar with vulkan but an uninitialized value is always suspect.

Improvements:

• Replace all enum constants with constexpr constants.
• Replace LOGLEVEL enum with an enum class.
• Remove some unused variables.
• Remove redundant const in some methods.
• Prefer using std::numeric_limits.
• Fix some missing initializers.