Add optional controller rumble on button press

New "Vibrate controller on button press" setting under Controls. When enabled,
pressing a gamepad button rumbles that pad for as long as it's held - something
the PSP hardware never had, but which some people want from a modern pad.

Implemented per platform behind System_ControllerRumbleStart/Stop: SDL uses
SDL_RumbleGamepad, Windows feeds it into XinputDevice's existing vibration path
so it doesn't fight with the vibration games request through sceCtrl, and
Android goes through InputDevice.getVibrator() in Java. Stubbed elsewhere.

The D-pad is excluded, since it gets tapped constantly while navigating.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01GgACRqkQNpfJQ4fjwyoEup

Windows/XinputDevice.cpp

@@ -2,6 +2,7 @@
 
 #include <climits>
 #include <algorithm>
+#include <atomic>
 
 #include "Common/System/NativeApp.h"
 #include "Common/CommonWindows.h"
@@ -123,6 +124,15 @@ static void UnloadXInputDLL() {}
 #define XUSER_MAX_COUNT 4
 #endif
 
+// Written from the input thread by XinputSetButtonRumble, read by ApplyVibration.
+static std::atomic<bool> g_buttonRumble[XUSER_MAX_COUNT];
+
+void XinputSetButtonRumble(int pad, bool on) {
+	if (pad >= 0 && pad < XUSER_MAX_COUNT) {
+		g_buttonRumble[pad] = on;
+	}
+}
+
 // Undocumented. Steam annoyingly grabs this button though....
 #define XINPUT_GUIDE_BUTTON 0x400
 
@@ -307,6 +317,7 @@ void XinputDevice::ApplyVibration(int pad, XINPUT_VIBRATION &vibration) {
 				vibration.wLeftMotorSpeed = 0;
 				vibration.wRightMotorSpeed = 0;
 			}
+			ApplyButtonRumble(pad, vibration);
 
 			if (padData_[pad].prevVibration.wLeftMotorSpeed != vibration.wLeftMotorSpeed || padData_[pad].prevVibration.wRightMotorSpeed != vibration.wRightMotorSpeed) {
 				PPSSPP_XInputSetState(pad, &vibration);
@@ -315,9 +326,19 @@ void XinputDevice::ApplyVibration(int pad, XINPUT_VIBRATION &vibration) {
 			prevVibrationTime_ = newVibrationTime_;
 		}
 	} else {
+		ApplyButtonRumble(pad, vibration);
 		DWORD dwResult = PPSSPP_XInputSetState(pad, &vibration);
 		if (dwResult != ERROR_SUCCESS) {
 			padData_[pad].checkDelayUpdates = 30;
 		}
 	}
 }
+
+void XinputDevice::ApplyButtonRumble(int pad, XINPUT_VIBRATION &vibration) {
+	// Runs the motors flat out for as long as the button is held, overriding whatever
+	// the game asked for - it's an explicit user setting, so it wins.
+	if (g_buttonRumble[pad]) {
+		vibration.wLeftMotorSpeed = 0xFFFF;
+		vibration.wRightMotorSpeed = 0xFFFF;
+	}
+}