IO: Added storage for PenPressure (unused by core library, to facilitate experiments) (#2372)

imgui.h

@@ -1584,6 +1584,7 @@ struct ImGuiIO
     float       KeysDownDurationPrev[512];      // Previous duration the key has been down
     float       NavInputsDownDuration[ImGuiNavInput_COUNT];
     float       NavInputsDownDurationPrev[ImGuiNavInput_COUNT];
+    float       PenPressure;                    // Touch/Pen pressure (0.0f to 1.0f, should be >0.0f only when MouseDown[0] == true). Helper storage currently unused by Dear ImGui.
     ImWchar16   InputQueueSurrogate;            // For AddInputCharacterUTF16
     ImVector<ImWchar> InputQueueCharacters;     // Queue of _characters_ input (obtained by platform back-end). Fill using AddInputCharacter() helper.