Fix: Previous key state for repeat count in X11

Currently, as we've spoken in aseprite#69
@dcap has correctly pointed out that my patch has some problems with
shift key modifier - and he was correct, I've skipped an important thing
looking into this problem.

This patch adds a std::set to hold all currently pressed keys - if a key
got pressed second time, std::set will hold a key symbol of that key, if
an event for that key comes for a second time (and there were no
KeyRelease event coming for the event in-between) - then it means the key is
being pressed down, so we set repeat count to 1. If KeyRelease event
came for the key symbol that was previously pressed, then we basically
erase it from std::set.

os/x11/window.cpp

@@ -34,6 +34,7 @@
 
 #include <array>
 #include <map>
+#include <set>
 
 #define KEY_TRACE(...)
 #define EVENT_TRACE(...)
@@ -115,6 +116,10 @@ std::map<::Window, WindowX11*> g_activeWindows;
 // for the XInput devices.
 Time g_lastXInputEventTime = 0;
 
+// Set of all currently pressed keys - used during processing KeyPress or KeyRelease
+// event
+std::set<KeySym> g_pressedKeys;
+
 bool is_mouse_wheel_button(int button)
 {
   return (button == Button4 || button == Button5 ||
@@ -1042,17 +1047,19 @@ void WindowX11::processX11Event(XEvent& event)
         KEY_TRACE("Xutf8LookupString %s\n", &buf[0]);
       }
 
-      // Check if the key has been pressed, 
-      // and if yes - check if it's the same one key
-      // as in the previous event.  If it's the same one
-      // - it means key is being held, so set repeat to 1.
+
+      // Check if the key has been pressed, and if yes - check what's
+      // the previous state of the key symbol - if it was previously
+      // pressed set repeat count of 1, if it wasn't pressed previously
+      // it will set repeat count to 0
       if (event.type == KeyPress) {
-        if (keysym == m_pressedKeySym)
+        if (g_pressedKeys.find(keysym) != g_pressedKeys.end())
           ev.setRepeat(1);
-        m_pressedKeySym = keysym;
+        else
+          g_pressedKeys.insert(keysym);
       }
       else
-        m_pressedKeySym = 0;
+        g_pressedKeys.erase(keysym);
 
       // Key event used by the input method (e.g. when the users
       // presses a dead key).

os/x11/window.h

@@ -128,8 +128,6 @@ class WindowX11 : public Window {
   bool m_resizable = false;
   bool m_transparent = false;
 
-  KeySym m_pressedKeySym = 0;
-
   // Double-click info
   Event::MouseButton m_doubleClickButton;
   base::tick_t m_doubleClickTick;