Crosses off a lingering TODO.

Also amends a few nearby cases where a u32 cast was being repromoted to
size_t.

Mostly involves moving contents of OGLRender
to OGLGfx and OGLConfig

This moves much of the duplicated bounding box code into VideoCommon,
leaving only the specific buffer implementations in each backend.

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.

We also need to ensure the the CPU does not receive stale values
which have been updated by the GPU. Apparently the buffer here
is not coherent on NVIDIA drivers. Not sure if this is a driver
bug/spec violation or not, one would think that
glGetBufferSubData() would invalidate any caches as needed, but
this path is only used on NVIDIA anyway, so it's fine. A point
to note is that according to ARB_debug_report, it's moved from
video to host memory, which would explain why it needs the
cache invalidate.