Crosses off a lingering TODO.

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

OpenGL: Check the list of supported AA modes instead of hardcoding

Before, it used a fallback where it returned a default object, where the width and height were set to 0. Presenter::Initialize() used GetSurfaceInfo to set the backbuffer size, then used that size when initializing the on-screen UI (even for the software renderer, where the on-screen UI isn't currently present), which meant that ImGui got a window size of 0 and thus resulted in a failed assertion.

Although BindBackbuffer checks for size changes, it doesn't help because ImGui has already been initialized, and the size hasn't actually changed since initialization occured.

Fixes one aspect of https://bugs.dolphin-emu.org/issues/13172.

Kill Renderer (with phire)

Not a good idea to abuse bSupportsPostProcessing

The whole ownership model was getting a bit of a mess, with a some
of special cases to deal with. And I'm planning to make it even more
complex in the future.
So here is some upfront work to convert it over to reference counted
pointers.