Future Dev 08: Program: fix redirection when loading preset from host

Hosts support saving current plugin state as user presets. And some
hosts like REAPER will set program index to 0 when loading them. At this
time, if plugin still load factory program #0, users won't get their own
presets. Such a "redirection" is unexpected.

One of the best solutions is: reserve factory program #0 as "initial
program". No matter when host or user set to this value, plugin state
will always stays untouched.

Then, correspondingly, move factory programs forward by one item. Always
keep in mind that programs displayed in host's menu is independent, as
it's enumerated by effGetProgramNameIndexed when loading plugin instance.

So, we can just let factory programs display from item amsynth#1, then when user
choose factory programs, reduce choice index by 1 in effSetProgram, and
this is the actual preset index kept by Amsynth preset manager.

src/amsynth_vst_imgui.cpp

@@ -159,9 +159,19 @@ static intptr_t dispatcher(AEffect *effect, int opcode, int index, intptr_t val,
 			// Stop all sound before applying preset
 			panic_on_dsp_side(effect);
 
+			// Program #0 is reserved for default / user-defined presets.
+			// Do not apply factory program at this time.
+			if (val <= 0) {
+				plugin->presetName = "Initial";
+				plugin->programNumber = 0;
+				return 1;
+			}
+
 			// Obtain bank and preset
-			auto &bank = PresetController::getPresetBanks().at(val / kPresetsPerBank);
-			auto &preset = bank.presets[val % kPresetsPerBank];
+			// The corresponding factory program index is just by one item before host's index.
+			int actual_index = val - 1;
+			auto &bank = PresetController::getPresetBanks().at(actual_index / kPresetsPerBank);
+			auto &preset = bank.presets[actual_index % kPresetsPerBank];
 
 			// Keep current preset name / index via our own variables
 			plugin->presetName = preset.getName();
@@ -199,12 +209,24 @@ static intptr_t dispatcher(AEffect *effect, int opcode, int index, intptr_t val,
 			// Host will query every built-in program by invoking effGetProgramNameIndexed.
 			// @param index: The program index where the host queries
 
+			// Reserve program #0 as default/customized program as "Initial",
+			// so that users can load user-defined presets from host instead of always being redirected to factory program #0.
+			// NOTE: Hosts like REAPER will set index to 0 when loading user-defined presets.
+			if (index <= 0) {
+				strncpy((char *)ptr, "Initial", 24);
+				return 1;
+			}
+
 			// Check if there were preset files or not
 			// If you don't check, host will crash if no preset was installed
 			if (PresetController::getPresetBanks().size()) {
+				// As #0 is "Initial", other factory programs should be put into the next hole.
+				// "index" is host's display index, and "actual_index" is the corresponding bank's index.
+				int actual_index = index - 1;
+
 				// Obtain bank and preset
-				auto &bank = PresetController::getPresetBanks().at(index / kPresetsPerBank);
-				auto &preset = bank.presets[index % kPresetsPerBank];
+				auto &bank = PresetController::getPresetBanks().at(actual_index / kPresetsPerBank);
+				auto &preset = bank.presets[actual_index % kPresetsPerBank];
 
 				// Set current obtained preset name, then show the name on host
 				plugin->presetName = preset.getName();