Merge branch 'viewport' into docking

# Conflicts:
#	imgui.cpp
#	imgui.h
#	imgui_internal.h

docs/CHANGELOG.txt

@@ -80,6 +80,11 @@ Other Changes:
   This affects clamping window within the visible area: with this option enabled title bars need to be visible. (#899)
 - Window: Fixed using SetNextWindowPos() on a child window (which wasn't really documented) position the cursor as expected
   in the parent window, so there is no mismatch between the layout in parent and the position of the child window. 
+- InputFloat: When using ImGuiInputTextFlags_ReadOnly the step buttons are disabled. (#2257)
+- Error recovery: Extraneous/undesired calls to End() are now being caught by an assert in the End() function itself 
+  at the call site (instead of being reported in EndFrame). Past the assert, they don't lead to crashes any more. (#1651)
+- Error recovery: Missing calls to End(), pass the assert, should not lead to crashes or to the fallback Debug window 
+  appearing on screen, (#1651).
 - IO: Added BackendPlatformUserData, BackendRendererUserData, BackendLanguageUserData void* for storage use by back-ends.
 - Style: Tweaked default value of style.DisplayWindowPadding from (20,20) to (19,19) so the default style as a value
   which is the same as the title bar height.

docs/README.md

@@ -236,7 +236,7 @@ The library started its life and is best known as "ImGui" only due to the fact t
 
 **How can I tell whether to dispatch mouse/keyboard to imgui or to my application?**
 <br>**How can I display an image? What is ImTextureID, how does it works?**
-<br>**How can I have multiple widgets with the same label or without a label? A primer on labels and the ID Stack.**
+<br>**How can I have multiple widgets with the same label or with an empty label? A primer on labels and the ID Stack.**
 <br>**How can I use my own math types instead of ImVec2/ImVec4?**
 <br>**How can I load a different font than the default?**
 <br>**How can I easily use icons in my application?**