Software: Fix bad backbuffer size

MAX_XFB_WIDTH/HEIGHT are the largest XFB sizes seen in practice, but do not make sense to use for the backbuffer size, which should be the size of the window.  The old code created screenshots with a size of 720x540 on NTSC games when "Dump Frames at Internal Resolution" is unchecked; now, the window size is used.

Source/Core/VideoBackends/Software/SWRenderer.cpp

@@ -27,7 +27,8 @@
 namespace SW
 {
 SWRenderer::SWRenderer(std::unique_ptr<SWOGLWindow> window)
-    : ::Renderer(static_cast<int>(MAX_XFB_WIDTH), static_cast<int>(MAX_XFB_HEIGHT), 1.0f,
+    : ::Renderer(static_cast<int>(std::max(window->GetContext()->GetBackBufferWidth(), 1u)),
+                 static_cast<int>(std::max(window->GetContext()->GetBackBufferHeight(), 1u)), 1.0f,
                  AbstractTextureFormat::RGBA8),
       m_window(std::move(window))
 {
@@ -56,6 +57,18 @@ SWRenderer::CreateFramebuffer(AbstractTexture* color_attachment, AbstractTexture
                                static_cast<SWTexture*>(depth_attachment));
 }
 
+void SWRenderer::BindBackbuffer(const ClearColor& clear_color)
+{
+  // Look for framebuffer resizes
+  if (!m_surface_resized.TestAndClear())
+    return;
+
+  GLContext* context = m_window->GetContext();
+  context->Update();
+  m_backbuffer_width = context->GetBackBufferWidth();
+  m_backbuffer_height = context->GetBackBufferHeight();
+}
+
 class SWShader final : public AbstractShader
 {
 public:

Source/Core/VideoBackends/Software/SWRenderer.h

@@ -27,6 +27,8 @@ public:
   std::unique_ptr<AbstractFramebuffer>
   CreateFramebuffer(AbstractTexture* color_attachment, AbstractTexture* depth_attachment) override;
 
+  void BindBackbuffer(const ClearColor& clear_color = {}) override;
+
   std::unique_ptr<AbstractShader> CreateShaderFromSource(ShaderStage stage,
                                                          std::string_view source) override;
   std::unique_ptr<AbstractShader> CreateShaderFromBinary(ShaderStage stage, const void* data,