Handle latch in dual MPE and no-23 mode

In MPE or ignore-23-to-scene mode, turning one scene
on to latch would mistakenly trigger voices in the non
latched scene. Fix that but adding an intended scene optional
argument to playNote and calling it from the right place. Also
add a regtest.

Closes surge-synthesizer#6963

src/common/SurgeSynthesizer.cpp

@@ -409,7 +409,7 @@ int SurgeSynthesizer::calculateChannelMask(int channel, int key)
 }
 
 void SurgeSynthesizer::playNote(char channel, char key, char velocity, char detune,
-                                int32_t host_noteid)
+                                int32_t host_noteid, int32_t forceScene)
 {
     if (halt_engine)
     {
@@ -464,6 +464,10 @@ void SurgeSynthesizer::playNote(char channel, char key, char velocity, char detu
     // MIDI Channel 3 plays B
 
     int channelmask = calculateChannelMask(channel, key);
+    if (forceScene == 0)
+        channelmask = 1;
+    if (forceScene == 1)
+        channelmask = 2;
 
     // TODO: FIX SCENE ASSUMPTION
     if (channelmask & 1)
@@ -4092,9 +4096,9 @@ void SurgeSynthesizer::processControl()
     // introduce int mods, we need to make sure the scenedata and so on is set up before
     // we latch
     if (playA && (storage.getPatch().scene[0].polymode.val.i == pm_latch) && voices[0].empty())
-        playNote(1, 60, 100, 0);
+        playNote(1, 60, 100, 0, -1, 0);
     if (playB && (storage.getPatch().scene[1].polymode.val.i == pm_latch) && voices[1].empty())
-        playNote(2, 60, 100, 0);
+        playNote(2, 60, 100, 0, -1, 1);
 
     for (int s = 0; s < n_scenes; s++)
     {

src/common/SurgeSynthesizer.h

@@ -82,8 +82,11 @@ class alignas(16) SurgeSynthesizer
     virtual ~SurgeSynthesizer();
 
     // Also see setNoteExpression() which allows you to control all note parameters polyphonically
-    // with the user-provided host_noteid parameter.
-    void playNote(char channel, char key, char velocity, char detune, int32_t host_noteid = -1);
+    // with the user-provided host_noteid parameter. forceScene means to ignore any channel
+    // related shenanigans and force you onto scene 0, 1, 2 etc... which is useful mostly
+    // for latch mode working in MPE and ignore 23 mode
+    void playNote(char channel, char key, char velocity, char detune, int32_t host_noteid = -1,
+                  int32_t forceScene = -1);
     void playNoteByFrequency(float freq, char velocity, int32_t id);
     void releaseNote(char channel, char key, char velocity, int32_t host_noteid = -1);
     void chokeNote(int16_t channel, int16_t key, char velocity, int32_t host_noteid = -1);

src/surge-testrunner/UnitTestsMIDI.cpp

@@ -1676,4 +1676,50 @@ TEST_CASE("Mono Modes Across Channels", "[midi]")
             }
         }
     }
+}
+
+TEST_CASE("Latch in Dual MPE", "[midi]")
+{
+    for (auto u23 : {false, true})
+    {
+        for (auto me : {false, true})
+        {
+            DYNAMIC_SECTION("Latch " << (me ? "With" : "Without")
+                                     << " MPE use23=" << (u23 ? "true" : "false"))
+            {
+                auto surge = surgeOnSine();
+                surge->storage.userDefaultsProvider->addOverride(
+                    Surge::Storage::UseCh2Ch3ToPlayScenesIndividually, u23);
+                surge->mpeEnabled = me;
+                surge->storage.getPatch().scenemode.val.i = sm_dual;
+
+                REQUIRE(surge->voices[0].empty());
+                REQUIRE(surge->voices[1].empty());
+
+                for (int i = 0; i < 10; ++i)
+                    surge->process();
+
+                REQUIRE(surge->voices[0].empty());
+                REQUIRE(surge->voices[1].empty());
+
+                auto &p = surge->storage.getPatch().scene[0].polymode;
+                auto id = surge->idForParameter(&p);
+
+                surge->setParameter01(
+                    id, Parameter::intScaledToFloat(pm_latch, p.val_max.i, p.val_min.i));
+                surge->process();
+
+                REQUIRE(surge->voices[0].size() == 1);
+                REQUIRE(surge->voices[1].size() == 0);
+
+                surge->setParameter01(
+                    id, Parameter::intScaledToFloat(pm_poly, p.val_max.i, p.val_min.i));
+                for (int i = 0; i < 100; ++i)
+                    surge->process();
+
+                REQUIRE(surge->voices[0].empty());
+                REQUIRE(surge->voices[1].empty());
+            }
+        }
+    }
 }