fix gamepad trigger range issue

Author: notimaginative

code/io/joy-sdl.cpp

@@ -975,10 +975,22 @@ namespace joystick
 	void Joystick::handleAxisEvent(const SDL_GamepadAxisEvent &evt)
 	{
 		auto axis = evt.axis;
+		auto value = evt.value;
 
 		Assertion(axis < numAxes(), "SDL event contained invalid axis index!");
 
-		_axisValues[axis] = evt.value;
+		// Triggers range from 0..32767 so we need to scale the value for FSO
+		// since it expects a full -32768..32767 range. Note that precision is
+		// lost in the scaling so only scale if not min/max trigger value
+		if ((axis == SDL_GAMEPAD_AXIS_LEFT_TRIGGER) || (axis == SDL_GAMEPAD_AXIS_RIGHT_TRIGGER)) {
+			if (value == 0) {
+				value = -32768;
+			} else if (value < 32767) {
+				value = static_cast<decltype(value)>((value * 2) - 32768);
+			}
+		}
+
+		_axisValues[axis] = value;
 	}
 
 	void Joystick::handleButtonEvent(const SDL_JoyButtonEvent &evt)