Software: Implement GetSurfaceInfo()

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.

Source/Core/VideoBackends/Null/NullGfx.h

@@ -34,6 +34,7 @@ public:
   std::unique_ptr<AbstractPipeline> CreatePipeline(const AbstractPipelineConfig& config,
                                                    const void* cache_data = nullptr,
                                                    size_t cache_data_length = 0) override;
+  SurfaceInfo GetSurfaceInfo() const override { return {}; }
 };
 
 class NullRenderer final : public Renderer

Source/Core/VideoBackends/Software/SWGfx.cpp

@@ -128,4 +128,11 @@ void SWGfx::SetScissorRect(const MathUtil::Rectangle<int>& rc)
   Rasterizer::ScissorChanged();
 }
 
+SurfaceInfo SWGfx::GetSurfaceInfo() const
+{
+  GLContext* context = m_window->GetContext();
+  return {std::max(context->GetBackBufferWidth(), 1u), std::max(context->GetBackBufferHeight(), 1u),
+          1.0f, AbstractTextureFormat::RGBA8};
+}
+
 }  // namespace SW

Source/Core/VideoBackends/Software/SWGfx.h

@@ -49,6 +49,8 @@ public:
   void ClearRegion(const MathUtil::Rectangle<int>& target_rc, bool colorEnable, bool alphaEnable,
                    bool zEnable, u32 color, u32 z) override;
 
+  SurfaceInfo GetSurfaceInfo() const override;
+
 private:
   std::unique_ptr<SWOGLWindow> m_window;
 };

Source/Core/VideoCommon/AbstractGfx.h

@@ -161,7 +161,7 @@ public:
   bool UseGeometryShaderForUI() const;
 
   // Returns info about the main surface (aka backbuffer)
-  virtual SurfaceInfo GetSurfaceInfo() const { return {}; }
+  virtual SurfaceInfo GetSurfaceInfo() const = 0;
 
 protected:
   AbstractFramebuffer* m_current_framebuffer = nullptr;