PostProcessing: Don't use GS expansion shader for quad buffering w/ O…

…penGL

OpenGL doesn't render to a 2-layer backbuffer like D3D/Vulkan for quad-buffered
stereo, instead drawing twice with the eye selected by glDrawBuffer()
(see OGL::Renderer::RenderXFBToScreen).

Source/Core/VideoCommon/PostProcessing.cpp

@@ -714,11 +714,15 @@ bool PostProcessing::CompilePixelShader()
 
 bool PostProcessing::CompilePipeline()
 {
+  // OpenGL doesn't render to a 2-layer backbuffer like D3D/Vulkan for quad-buffered stereo, instead
+  // drawing twice and the eye selected by glDrawBuffer() (see OGL::Renderer::RenderXFBToScreen).
+  const bool use_quad_buffer_gs = g_ActiveConfig.stereo_mode == StereoMode::QuadBuffer &&
+                                  g_ActiveConfig.backend_info.api_type != APIType::OpenGL;
+
   AbstractPipelineConfig config = {};
   config.vertex_shader = m_vertex_shader.get();
-  config.geometry_shader = g_ActiveConfig.stereo_mode == StereoMode::QuadBuffer ?
-                               g_shader_cache->GetTexcoordGeometryShader() :
-                               nullptr;
+  config.geometry_shader =
+      use_quad_buffer_gs ? g_shader_cache->GetTexcoordGeometryShader() : nullptr;
   config.pixel_shader = m_pixel_shader.get();
   config.rasterization_state = RenderState::GetNoCullRasterizationState(PrimitiveType::Triangles);
   config.depth_state = RenderState::GetNoDepthTestingDepthState();