Fix some common anti-patterns with these data structures.
- You can dereference the iterator returned by `find` to access the
underlying value directly, without an extra `operator[]`/`at`.
- Rather than checking for an element before insertion/deletion, you can
just do the operation and if needed check the return value to
determine if the insertion/deletion succeeded.
Instead, we make the event take a reference to the system and then pass
it in when the event is triggered.
This does introduce two other accessors, but these are much easier to
refactor out over time, and without modification to the existing event
interface.
Almost all the virtual functions in Renderer are part of dolphin's
"graphics api abstraction layer", which has slowly formed over the
last decade or two.
Most of the work was done previously with the introduction of the
various "AbstractX" classes, associated with texture cache cleanups
and implementation of newer graphics APIs (Direct3D 12, Vulkan, Metal).
We are simply taking the last step and yeeting these functions out
of Renderer.
This "AbstractGfx" class is now completely agnostic of any details
from the flipper/hollywood GPU we are emulating, though somewhat
specialized.
(Will not build, this commit only contains changes outside VideoBackends)
