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.

backends/imgui_impl_vulkan.cpp

@@ -2,8 +2,8 @@
 // This needs to be used along with a Platform Backend (e.g. GLFW, SDL, Win32, custom..)
 
 // Implemented features:
-//  [X] Renderer: Support for large meshes (64k+ vertices) with 16-bit indices.
 //  [!] Renderer: User texture binding. Use 'VkDescriptorSet' as ImTextureID. Read the FAQ about ImTextureID! See https://github.com/ocornut/imgui/pull/914 for discussions.
+//  [X] Renderer: Large meshes support (64k+ vertices) with 16-bit indices.
 
 // Important: on 32-bit systems, user texture binding is only supported if your imconfig file has '#define ImTextureID ImU64'.
 // This is because we need ImTextureID to carry a 64-bit value and by default ImTextureID is defined as void*.
@@ -117,7 +117,7 @@ struct ImGui_ImplVulkan_Data
     VkDeviceMemory              UploadBufferMemory;
     VkBuffer                    UploadBuffer;
 
-    // Render buffers
+    // Render buffers for main window
     ImGui_ImplVulkanH_WindowRenderBuffers MainWindowRenderBuffers;
 
     ImGui_ImplVulkan_Data()