Fixes a build failure with clang 17.
The destructor needs to be in the cpp file, since we have a forward
declared std::unique_ptr type as part of the class. So technically the
default inline destructor could invoke without seeing the full data type
definition.
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.
TellowKrinkle