Fixes https://bugs.dolphin-emu.org/issues/13136.

This assertion is currently triggered by Pocoyo Racing (https://bugs.dolphin-emu.org/issues/13136).

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.

Migrates off the printf-based formatting where applicable.

Now that we've extracted all of the stateless functions that can be
hidden, it's time to make the index generator a regular class with
active data members.

This can just be a member that sits within the vertex manager base
class. By deglobalizing the state of the index generator we also get rid
of the wonky dual-initializing that was going on within the OpenGL
backend.

Since the renderer is always initialized before the vertex manager, we
now only call Init() once throughout the execution lifecycle.

We can use if constexpr with the template functions that pass in a
non-type template parameter, allowing the removal of branches that
aren't taken at compile time.

Compilers will generally do this by default, however, we now give a
gentle prodding to the compiler if this would otherwise not be the case.

These don't rely on any of the static members within the IndexGenerator
class, so we can make all of these functions fully internal to the
translation unit.