VideoBackends / VideoCommon: add support for specifying include files in shader code

Fixes a crash that could occur if the static constructor function for
the MainSettings.cpp TU happened to run before the variables in
Common/Version.cpp are initialised. (This is known as the static
initialisation order fiasco.)

By using wrapper functions, those variables are now guaranteed to be
constructed on first use.

Note that D3DCommon can't use DX11HRWrap or DX12HRWrap since it's shared between them.

SPDX standardizes how source code conveys its copyright and licensing
information. See https://spdx.github.io/spdx-spec/1-rationale/ . SPDX
tags are adopted in many large projects, including things like the Linux
kernel.

Converts lingering panic alert calls over to the fmt-capable ones.