video_core: NVIDIA DLSS through NVIDIA Streamline in place of a game's DLSS

Adds a Graphics setting, "Use NVIDIA DLSS in place of a game's DLSS" (nvidia_dlss, off by
default). When it is on, in Windows builds configured with EGLANTINE_STREAMLINE_SDK, the
evaluation Eglantine already replaces with FSR 3.1 goes to NVIDIA DLSS on GeForce RTX cards, with
the same inputs (colour, depth, motion vectors, jitter). When DLSS can't run, FSR 3.1 runs as
before.

Only Streamline's MIT-licensed headers are compiled in; nothing of NVIDIA's is linked or shipped.
At run time Eglantine looks for sl.interposer.dll in the user folder's "streamline" folder (or
EDEN_STREAMLINE_DIR, or "streamline" next to the executable), checks its Streamline signature and
starts Streamline with manual hooking. Streamline's plugins come from that folder or the NVIDIA
app's downloads. DLSS itself is nvngx_dlss.dll in the same folder, else the newest DLSS 310 or
later that the NVIDIA app has downloaded, checked for NVIDIA's Authenticode signature and copied
as streamline/nvidia-app/nvngx_dlss.dll. Like other tools that call DLSS outside a game, it uses
its own fixed project id with the custom engine type.

- Preset K for all modes (EDEN_NX2_DLSS_PRESET overrides it): the default Performance model (M)
  and L draw thin dark horizontal lines in some frames at 2x resolution scale; K doesn't.
- HDR input with auto exposure; the mode follows the render/output ratio.
- Buffer device address stays enabled when DLSS needs it, for DLSS only.
- The upscaler waits for the device instead of submitting when it is destroyed: by then the
  game's channels are released, and a submit read the state tracker's stale flags (a crash when
  stopping a game).

With the setting off, or in builds without the SDK path (the setting is hidden there), nothing
changes.

Testing: Windows 11, RTX 5080, Fire Emblem: Fortune's Weave docked at 1x (720p -> 1440p) and 2x
(1440p -> 2880p), CLI runs and GUI play, stopping and restarting games with DLSS on. Not tested on
other GPUs or games.

Assisted-by: Claude Opus 5.5 (implementation, debugging)
Signed-off-by: Duck912 <350528965+Duck912@users.noreply.github.com>

4 files changed, 5 insertions, 0 deletions

diff --git a/src/common/fs/fs_paths.h b/src/common/fs/fs_paths.h
index ae25846fc2..763a8759dd 100644
--- a/src/common/fs/fs_paths.h
+++ b/src/common/fs/fs_paths.h
@@ -28,6 +28,7 @@
 #define SCREENSHOTS_DIR "screenshots"
 #define SDMC_DIR "sdmc"
 #define SHADER_DIR "shader"
+#define STREAMLINE_DIR "streamline"
 #define TAS_DIR "tas"
 #define ICONS_DIR "icons"
 
diff --git a/src/common/fs/path_util.cpp b/src/common/fs/path_util.cpp
index 82ea8f6cb2..f77da2e301 100644
--- a/src/common/fs/path_util.cpp
+++ b/src/common/fs/path_util.cpp
@@ -168,6 +168,7 @@ public:
         GenerateEdenPath(EdenPath::SDMCDir, eden_path / SDMC_DIR);
         GenerateEdenPath(EdenPath::TASDir, eden_path / TAS_DIR);
         GenerateEdenPath(EdenPath::IconsDir, eden_path / ICONS_DIR);
+        GenerateEdenPath(EdenPath::StreamlineDir, eden_path / STREAMLINE_DIR);
         // config
         GenerateEdenPath(EdenPath::ConfigDir, eden_path_config);
         // cache
diff --git a/src/common/fs/path_util.h b/src/common/fs/path_util.h
index e8fa5614ae..940dbb9186 100644
--- a/src/common/fs/path_util.h
+++ b/src/common/fs/path_util.h
@@ -34,6 +34,7 @@ enum class EdenPath {
     ShaderDir,      // Where shaders are stored.
     TASDir,         // Where TAS scripts are stored.
     IconsDir,       // Where Icons for Windows shortcuts are stored.
+    StreamlineDir,  // Where the user-supplied NVIDIA Streamline loader is stored.
 };
 
 // migration/compat dirs
diff --git a/src/common/settings.h b/src/common/settings.h
index 6669f98f4c..dc442a7180 100644
--- a/src/common/settings.h
+++ b/src/common/settings.h
@@ -378,6 +378,8 @@ struct Values {
                                                            Specialization::Percentage,
                                                        true,
                                                        true};
+    // NVIDIA DLSS (through NVIDIA Streamline) in place of a game's DLSS.
+    SwitchableSetting<bool> nvidia_dlss{linkage, false, "nvidia_dlss", Category::Renderer};
     SwitchableSetting<AspectRatio, true> aspect_ratio{linkage,
                                                       AspectRatio::R16_9,
                                                       "aspect_ratio",