Frame generation: in-between frames never rasterize native VRAM

Under HD texture-pack authority (and dual-raster netplay) every draw was
also rasterized into CPU VRAM unless a render pass was active. Replayed
draws of a generated frame were not exempt, so each in-between frame wrote
interpolated geometry into the guest's native VRAM. They are presentation
only, like a pass. Fixture: compose under HD authority, VRAM unchanged.

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

runtime/src/gpu_gl_renderer.c

@@ -4291,8 +4291,9 @@ static int cpu_raster_required(void) {
      * GPU surface. The CPU mirror stays the authoritative record of the real
      * frame, so the pass never rasterizes into it. depth24 scanout still
      * presents from the CPU mirror and keeps its write-through. */
+    /* A generated (in-between) frame is presentation only, like a pass. */
     return !s_raster_ok || s_depth24_skip_up ||
-           ((s_cpu_auth_dual || s_hd_native_authority) && !s_pass_active);
+           ((s_cpu_auth_dual || s_hd_native_authority) && !s_pass_active && !s_fg_drawing);
 }
 /* Bracket each authoritative draw so a later netplay/session policy change
  * cannot accidentally disable the faithful floor for HD replacement mode. */

runtime/tests/test_gl_frame_gen.c

@@ -354,6 +354,15 @@ int main(int argc, char **argv) {
             FILE *f = fopen("pm.rgba", "wb"); fwrite(img, 4, (size_t)ww * wh, f); fclose(f);
         }
         check(dm != r0 && dm != r1, "phase 0.5 is neither endpoint");
+        /* With an HD texture pack the CPU raster is native VRAM's authority;
+         * an in-between frame is presentation only and must not draw there. */
+        gl_renderer_set_hd_texture_mode(1);
+        static uint16_t before[1024 * 512];
+        memcpy(before, s_vram, sizeof before);
+        check(fg_generate(0.5, 0) == 1, "compose at phase 0.5 under HD authority");
+        check(memcmp(before, s_vram, sizeof before) == 0,
+              "generated frame leaves native VRAM unchanged under HD authority");
+        gl_renderer_set_hd_texture_mode(0);
         free(img);
         check(n_gen > 0, "generated frames were presented");
     }