PostProcessing/FX: Alternate between two 'backbuffers'

Saves video memory, and also fixes shaders that expect each pass's input
to be the output of the previous path without explicitly referencing the
render target.

src/util/postprocessing_shader_fx.cpp

@@ -1161,6 +1161,11 @@ bool PostProcessing::ReShadeFXShader::CreatePasses(GPUTexture::Format backbuffer
     m_textures.push_back(std::move(tex));
   }
 
+  // need potentially up to two backbuffers
+  std::array<std::optional<TextureID>, 2> backbuffer_texture_ids;
+  std::optional<TextureID> read_backbuffer;
+  u32 current_backbuffer = 0;
+
   for (const reshadefx::technique& tech : mod.techniques)
   {
     for (const reshadefx::pass& pi : tech.passes)
@@ -1202,14 +1207,28 @@ bool PostProcessing::ReShadeFXShader::CreatePasses(GPUTexture::Format backbuffer
       }
       else
       {
-        Texture new_rt;
-        new_rt.format = backbuffer_format;
-        new_rt.render_target = true;
-        new_rt.storage_access = false;
-        new_rt.render_target_width = 0;
-        new_rt.render_target_height = 0;
-        pass.render_targets.push_back(static_cast<TextureID>(m_textures.size()));
-        m_textures.push_back(std::move(new_rt));
+        // swap to the other backbuffer, sample from the previous written
+        if (backbuffer_texture_ids[current_backbuffer].has_value())
+        {
+          read_backbuffer = backbuffer_texture_ids[current_backbuffer];
+          current_backbuffer ^= 1;
+        }
+
+        if (!backbuffer_texture_ids[current_backbuffer].has_value())
+        {
+          Texture new_rt;
+          new_rt.format = backbuffer_format;
+          new_rt.render_target = true;
+          new_rt.storage_access = false;
+          new_rt.render_target_width = 0;
+          new_rt.render_target_height = 0;
+          new_rt.reshade_name = fmt::format("| BackBuffer{} |", current_backbuffer);
+
+          backbuffer_texture_ids[current_backbuffer] = static_cast<TextureID>(m_textures.size());
+          m_textures.push_back(std::move(new_rt));
+        }
+
+        pass.render_targets.push_back(backbuffer_texture_ids[current_backbuffer].value());
       }
 
       u32 texture_slot = 0;
@@ -1232,7 +1251,7 @@ bool PostProcessing::ReShadeFXShader::CreatePasses(GPUTexture::Format backbuffer
             // found the texture, now look for our side of it
             if (ti.semantic == "COLOR")
             {
-              sampler.texture_id = INPUT_COLOR_TEXTURE;
+              sampler.texture_id = read_backbuffer.value_or(INPUT_COLOR_TEXTURE);
               break;
             }
             else if (ti.semantic == "DEPTH")