Merge misc changes from docking branch to reduce small drift.

In particular:
- imgui.cpp : move UpdateInputEvents() higher in NewFrame() to match docking + update RenderMouseCursor() to match.
- imgui_draw.cpp: ImDrawList::_ResetForNewFrame() change from c807192ab
- Backends: SDL2. Add MouseWindowID + change SDL_CaptureMouse() test to match docking branch. Not strictly necessary but aimed at reducing drift because we go on and fork this file.
+ moved responsability of checking valid names to TabBarGetTabName() to simplify both branches.

imgui_draw.cpp

@@ -389,6 +389,8 @@ void ImDrawList::_ResetForNewFrame()
     IM_STATIC_ASSERT(IM_OFFSETOF(ImDrawCmd, ClipRect) == 0);
     IM_STATIC_ASSERT(IM_OFFSETOF(ImDrawCmd, TextureId) == sizeof(ImVec4));
     IM_STATIC_ASSERT(IM_OFFSETOF(ImDrawCmd, VtxOffset) == sizeof(ImVec4) + sizeof(ImTextureID));
+    if (_Splitter._Count > 1)
+        _Splitter.Merge(this);
 
     CmdBuffer.resize(0);
     IdxBuffer.resize(0);