Fixed GetKeyName() for ImGuiMod_XXX values, made invalid MousePos display in log nicer.  (#4921, #456)

Amend fd408c9
diff --git a/imgui.cpp b/imgui.cpp
index 69efe64..80710fa 100644
--- a/imgui.cpp
+++ b/imgui.cpp
@@ -7817,6 +7817,8 @@
 #endif
     if (key == ImGuiKey_None)
         return "None";
+    if (key & ImGuiMod_Mask_)
+        key = ConvertSingleModFlagToKey(key);
     if (!IsNamedKey(key))
         return "Unknown";
 
@@ -8086,7 +8088,7 @@
 static void DebugPrintInputEvent(const char* prefix, const ImGuiInputEvent* e)
 {
     ImGuiContext& g = *GImGui;
-    if (e->Type == ImGuiInputEventType_MousePos)    { IMGUI_DEBUG_LOG_IO("%s: MousePos (%.1f, %.1f)\n", prefix, e->MousePos.PosX, e->MousePos.PosY); return; }
+    if (e->Type == ImGuiInputEventType_MousePos)    { if (e->MousePos.PosX == -FLT_MAX && e->MousePos.PosY == -FLT_MAX) IMGUI_DEBUG_LOG_IO("%s: MousePos (-FLT_MAX, -FLT_MAX)\n", prefix); else IMGUI_DEBUG_LOG_IO("%s: MousePos (%.1f, %.1f)\n", prefix, e->MousePos.PosX, e->MousePos.PosY); return; }
     if (e->Type == ImGuiInputEventType_MouseButton) { IMGUI_DEBUG_LOG_IO("%s: MouseButton %d %s\n", prefix, e->MouseButton.Button, e->MouseButton.Down ? "Down" : "Up"); return; }
     if (e->Type == ImGuiInputEventType_MouseWheel)  { IMGUI_DEBUG_LOG_IO("%s: MouseWheel (%.1f, %.1f)\n", prefix, e->MouseWheel.WheelX, e->MouseWheel.WheelY); return; }
     if (e->Type == ImGuiInputEventType_Key)         { IMGUI_DEBUG_LOG_IO("%s: Key \"%s\" %s\n", prefix, ImGui::GetKeyName(e->Key.Key), e->Key.Down ? "Down" : "Up"); return; }
@@ -8205,7 +8207,7 @@
         }
         else if (e->Type == ImGuiInputEventType_Focus)
         {
-            // We intentionally overwrite this and process lower, in order to give a chance
+            // We intentionally overwrite this and process in NewFrame(), in order to give a chance
             // to multi-viewports backends to queue AddFocusEvent(false) + AddFocusEvent(true) in same frame.
             const bool focus_lost = !e->AppFocused.Focused;
             e->IgnoredAsSame = (io.AppFocusLost == focus_lost);