This was very likely intended to be a logical OR based off the
conditioning and testing of inversion in one case.
Even if this was intentional, this is the kind of non-obvious thing one
should be clarifying with a comment.
- Fixed all warnings, for renderer_opengl items, which were indicating a
possible incorrect behavior from integral promotion rules and types
larger than those in which arithmetic is typically performed.
- Added const for variables where possible and meaningful.
- Added constexpr where possible.
Namespaces all OpenGL code under the OpenGL namespace.
Prevents polluting the global namespace and allows clear distinction
between other renderers' code in the future.
