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.
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)
