SDL: On wayland use SDL_GetMouseState (#2802)

When the SDL Video backend is set to wayland the function
`SDL_GetGlobalMouseState` does not work. Most probably a security measure
for programs to not grab everything the user does
https://wiki.libsdl.org/SDL_GetGlobalMouseState

using SDL_GetMouseState https://wiki.libsdl.org/SDL_GetMouseState works
on wayland

examples/imgui_impl_sdl.cpp

@@ -17,6 +17,7 @@
 
 // CHANGELOG
 // (minor and older changes stripped away, please see git history for details)
+//  2019-12-17: Inputs: Use SDL_GetMouseState, because there is no global mouse state on Wayland.
 //  2019-12-05: Inputs: Added support for ImGuiMouseCursor_NotAllowed mouse cursor.
 //  2019-07-21: Inputs: Added mapping for ImGuiKey_KeyPadEnter.
 //  2019-04-23: Inputs: Added support for SDL_GameController (if ImGuiConfigFlags_NavEnableGamepad is set by user application).
@@ -60,6 +61,7 @@ static Uint64       g_Time = 0;
 static bool         g_MousePressed[3] = { false, false, false };
 static SDL_Cursor*  g_MouseCursors[ImGuiMouseCursor_COUNT] = {};
 static char*        g_ClipboardTextData = NULL;
+static bool         g_VideoDriverIsWayland = false;
 
 static const char* ImGui_ImplSDL2_GetClipboardText(void*)
 {
@@ -124,6 +126,9 @@ static bool ImGui_ImplSDL2_Init(SDL_Window* window)
 {
     g_Window = window;
 
+    // Check and store if we are on Wayland to not check on every frame.
+    g_VideoDriverIsWayland = strncmp(SDL_GetCurrentVideoDriver(), "wayland", 7) == 0;
+
     // Setup back-end capabilities flags
     ImGuiIO& io = ImGui::GetIO();
     io.BackendFlags |= ImGuiBackendFlags_HasMouseCursors;       // We can honor GetMouseCursor() values (optional)
@@ -237,14 +242,18 @@ static void ImGui_ImplSDL2_UpdateMousePosAndButtons()
 #if SDL_HAS_CAPTURE_AND_GLOBAL_MOUSE && !defined(__EMSCRIPTEN__) && !defined(__ANDROID__) && !(defined(__APPLE__) && TARGET_OS_IOS)
     SDL_Window* focused_window = SDL_GetKeyboardFocus();
     if (g_Window == focused_window)
+    {
+        if (!g_VideoDriverIsWayland)
         {
             // SDL_GetMouseState() gives mouse position seemingly based on the last window entered/focused(?)
             // The creation of a new windows at runtime and SDL_CaptureMouse both seems to severely mess up with that, so we retrieve that position globally.
+            // Won't use this workaround when on Wayland, as there is no global mouse position.
             int wx, wy;
             SDL_GetWindowPosition(focused_window, &wx, &wy);
             SDL_GetGlobalMouseState(&mx, &my);
             mx -= wx;
             my -= wy;
+        }
         io.MousePos = ImVec2((float)mx, (float)my);
     }