Present thread: one owned swap call; frame-gen fixture builds on MinGW

Both swap paths go through gl_window_swap. The temporal-blending ownership
guard allows the present thread's context (pt_begin only). The frame
generation fixture used setenv, which MinGW lacks.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

runtime/src/gpu_gl_renderer.c

@@ -8301,6 +8301,10 @@ static int pt_cb_ctx(void *user, int current) {
     return 1;
 }
 
+/* The one owned swap call: the composing thread's (direct) or the present
+ * thread's. */
+static void gl_window_swap(void) { SDL_GL_SwapWindow(s_win); }
+
 static void *pt_cb_present(void *user, int slot, void *ready) {
     (void)user;
     if (s_ptp_interval_gen != s_pt_interval_gen) {
@@ -8335,7 +8339,7 @@ static void *pt_cb_present(void *user, int slot, void *ready) {
     GL_PRESENT_THREAD_TEST_HOOK(slot);
 #endif
     void *done = p_glFenceSync(PT_SYNC_GPU_COMMANDS_COMPLETE, 0);
-    SDL_GL_SwapWindow(s_win);
+    gl_window_swap();
     return done;
 }
 
@@ -8412,7 +8416,7 @@ static void pt_end(void) {
 
 /* The swap: direct, or queued to the present thread. */
 static void gl_present_swap(void) {
-    if (!s_pt_on) { SDL_GL_SwapWindow(s_win); return; }
+    if (!s_pt_on) { gl_window_swap(); return; }
     (void)pt_target_fbo();   /* a frame that never bound 0 still has its slot */
     void *ready = p_glFenceSync(PT_SYNC_GPU_COMMANDS_COMPLETE, 0);
     glFlush();

runtime/tests/test_frame_interpolation_context_ownership.py

@@ -9,9 +9,15 @@ RENDERER = (ROOT / "runtime" / "src" / "gpu_gl_renderer.c").read_text(
     encoding="utf-8"
 )
 
-assert RENDERER.count("SDL_GL_CreateContext(") == 1, (
+# The only second context is the present thread's ([video] present_thread,
+# docs/RENDER_THREAD.md), created in pt_begin under the render thread, which
+# frame interpolation makes ineligible.
+assert RENDERER.count("SDL_GL_CreateContext(") == 2, (
     "temporal blending must not create a second GL context"
 )
+_pt = RENDERER[RENDERER.index("static int pt_begin(void)"):]
+_pt = _pt[:_pt.index("\n}\n")]
+assert "SDL_GL_CreateContext(" in _pt, "the second GL context belongs to the present thread"
 assert "SDL_CreateThread(" not in RENDERER, (
     "temporal blending must not create a presentation thread"
 )

runtime/tests/test_gl_frame_gen.c

@@ -254,7 +254,11 @@ int main(int argc, char **argv) {
     check(gl_renderer_render_thread_start(2) == 1, "render thread started");
     if (pt_mode) check(gl_renderer_present_thread_active(), "present thread started");
     if (fg) {
+#ifdef _WIN32
+        _putenv("PSX_FRAME_GEN_FORCE=1");
+#else
         setenv("PSX_FRAME_GEN_FORCE", "1", 1);
+#endif
         gl_renderer_set_frame_generation(1);
         check(gl_renderer_frame_generation() == 1, "frame generation on");
     }