Fix capture.get_jpg blocking during game load for Companion Mirror (#750)

Add a timeout and skip signalling to the D3D9 capture path so the API
thread no longer waits indefinitely when Present stops during loading.
Return the last successful JPEG frame as a fallback.

## Link to GitHub Issue or related Pull Request, if one exists
#746

## Description of change
* Fix `capture.get_jpg` blocking indefinitely when D3D9 screen capture
cannot
  complete during game loading (e.g. no Present calls).
* Add a 2-second timeout to `graphics_capture_receive_jpeg()` and cancel
  pending capture requests on timeout.
* Signal capture skip from D3D9 failure paths via
`graphics_capture_skip()`.
* Cache the last successful JPEG per screen and return it as a fallback
  when a new capture fails.
Tested with **stock iOS Spice Companion** only. I have not tested
Android or
other Companion clients. The iOS client implementation differs from the
others; this fix is server-side only.
No config file, CLI option, or API schema changes.

## Testing
* Built locally with llvm-mingw cross-compiler (WSL).
* Docker build (`src/spice2x/build_docker.sh`): Pending
* Tested with stock iOS Spice Companion on iPad: Mirror remains
connected
  during game loading instead of returning to KeyPad.

src/spice2x/api/modules/capture.cpp

@@ -1,5 +1,7 @@
 #include "capture.h"
 #include <functional>
+#include <mutex>
+#include <unordered_map>
 #include "external/rapidjson/document.h"
 #include "hooks/graphics/graphics.h"
 #include "util/crypt.h"
@@ -14,6 +16,56 @@ namespace api::modules {
 
     static thread_local std::vector<uint8_t> CAPTURE_BUFFER;
 
+    struct CachedFrame {
+        std::vector<uint8_t> jpeg;
+        uint64_t timestamp = 0;
+        int width = 0;
+        int height = 0;
+    };
+
+    static std::mutex FRAME_CACHE_M;
+    static std::unordered_map<int, CachedFrame> FRAME_CACHE;
+
+    static void add_jpeg_response(
+            int screen,
+            uint64_t timestamp,
+            int width,
+            int height,
+            const std::vector<uint8_t> &jpeg,
+            Response &res) {
+
+        auto encoded = crypt::base64_encode(jpeg.data(), jpeg.size());
+
+        Value data;
+        data.SetString(encoded.c_str(), encoded.length(), res.doc()->GetAllocator());
+        res.add_data(timestamp);
+        res.add_data(width);
+        res.add_data(height);
+        res.add_data(data);
+
+        std::lock_guard<std::mutex> lock(FRAME_CACHE_M);
+        FRAME_CACHE[screen] = {jpeg, timestamp, width, height};
+    }
+
+    static bool try_cached_response(int screen, Response &res) {
+        std::lock_guard<std::mutex> lock(FRAME_CACHE_M);
+        const auto pos = FRAME_CACHE.find(screen);
+        if (pos == FRAME_CACHE.end() || pos->second.jpeg.empty()) {
+            return false;
+        }
+
+        const auto &cached = pos->second;
+        auto encoded = crypt::base64_encode(cached.jpeg.data(), cached.jpeg.size());
+
+        Value data;
+        data.SetString(encoded.c_str(), encoded.length(), res.doc()->GetAllocator());
+        res.add_data(cached.timestamp);
+        res.add_data(cached.width);
+        res.add_data(cached.height);
+        res.add_data(data);
+        return true;
+    }
+
     Capture::Capture() : Module("capture") {
         functions["get_screens"] = std::bind(&Capture::get_screens, this, _1, _2);
         functions["get_jpg"] = std::bind(&Capture::get_jpg, this, _1, _2);
@@ -41,6 +93,7 @@ namespace api::modules {
      * reduce: uint for dividing image size
      */
     void Capture::get_jpg(Request &req, Response &res) {
+        CAPTURE_BUFFER.clear();
         CAPTURE_BUFFER.reserve(1024 * 128);
 
         // settings
@@ -71,24 +124,15 @@ namespace api::modules {
         bool success = graphics_capture_receive_jpeg(screen, [] (uint8_t byte) {
             CAPTURE_BUFFER.push_back(byte);
         }, true, quality, true, divide, &timestamp, &width, &height);
-        if (!success) {
+
+        if (success) {
+            add_jpeg_response(screen, timestamp, width, height, CAPTURE_BUFFER, res);
+            CAPTURE_BUFFER.clear();
             return;
         }
 
-        // encode to base64
-        auto encoded = crypt::base64_encode(
-                CAPTURE_BUFFER.data(),
-                CAPTURE_BUFFER.size());
-
-        // clear buffer
+        // fall back to the last successful frame while the game is busy loading
         CAPTURE_BUFFER.clear();
-
-        // add data to response
-        Value data;
-        data.SetString(encoded.c_str(), encoded.length(), res.doc()->GetAllocator());
-        res.add_data(timestamp);
-        res.add_data(width);
-        res.add_data(height);
-        res.add_data(data);
+        try_cached_response(screen, res);
     }
 }