[visual] Change pressure sensitivity from linear to exponential

The default dependence of thickness on pressure is linear; this gives a
weird feel (to me).  Turn it into an exponential relation.

src/core/gui/inputdevices/AbstractInputHandler.cpp

@@ -7,6 +7,7 @@
 #include "gui/XournalppCursor.h"
 
 #include "InputContext.h"
+#include <cmath>
 
 AbstractInputHandler::AbstractInputHandler(InputContext* inputContext) { this->inputContext = inputContext; }
 
@@ -69,7 +70,7 @@ auto AbstractInputHandler::getInputDataRelativeToCurrentPage(XojPageView* page,
     pos.pressure = Point::NO_PRESSURE;
 
     if (this->inputContext->getSettings()->isPressureSensitivity()) {
-        pos.pressure = event.pressure;
+        pos.pressure = 1-exp(- event.pressure/0.2);
     }
 
     pos.state = this->inputContext->getModifierState();