Also fixes addressing of MSAA samples 2 and 3 for 64bpp color render targets in the ROV RB implementation on Direct3D 12.
Additionally, with FSI/ROV, alpha test and alpha to coverage are done only if the render target 0 was dynamically written to (according to the Direct3D 9 rules for writing to color render targets, though not sure if they actually apply to the alpha tests on Direct3D 9, but for safety).
There is also some code cleanup for things spotted during the development of the feature.

The pkg_config helper for premake was not checking for errors. When it
failed to run, either when it is not installed or the queried package is
not found, a cryptic error message would be printed:

"Error: .../xenia/third_party/premake-core/src/base/string.lua:36: attempt to index a nil value (upvalue 's')"

Fix this by checking the return status when calling pkg-config and
printing a descriptive error message.

Fixes #1868