Various tweaks and fixes as suggested by PVS Studio (thanks PVS Studio!)

Docking: Demo: Fixed docking document window into parent window. (#2286)

Merge branch 'viewport' into docking

Merge branch 'master' into viewport ...

# Conflicts:
#	docs/CHANGELOG.txt
#	examples/imgui_impl_win32.cpp
#	examples/imgui_impl_win32.h

Viewport: Made platform_io.Monitors mandatory for proper multi-viewport use.

Examples: Win32: Added support for XInput games (if ImGuiConfigFlags_NavEnableGamepad is enabled).

Examples: Win32: Using GetForegroundWindow() instead of GetActiveWindow() to be compatible with windows created in a different thread. (#1951, #2087, #2156, #2232) [many people]

Version 1.68 WIP

Merge branch 'viewport' into docking ...

# Conflicts:
#	docs/TODO.txt

Merge branch 'master' into viewport

TODO update + internals: changed order or ImGuiLayoutType enums to allow using them for indexing.

Fixed cursor issue caused by 6890e08b when calling BeginChild/EndChild multiple times to reappend into a same child window. (#2282)

Merge branch 'viewport' into docking

Merge branch 'master' into viewport

Version 1.67

DragFloat: Fixed broken mouse direction change with power!=1.0. (#2174, #2206) [@Joshhua5]

Added a bunch of diagnostic ignore to cope with Clang -Weverything being absurd. Also fixed two legit warnings. (#2277)

Merge branch 'viewport' into docking

Merge branch 'master' into viewport

Merge branch 'atlas_fixes'

Demo: Fixed bounds of DragFloat in Clipping section to avoid passing zero-sized to InvisibleButton().

FAQ entry, moved ImTextureId, Gallery links.

imgui_freetype: Fixed redundant FT_Load_Glyph() calls, unused parameters, and compilation warning/error. (#2270)

ImFontAtlas: Fixed allocating for last bit (would only affect is that last codepoint is a multiple of 32). (#2270)

ImFontAtlas: Rewrote FreeType based builder. ...

- Fixed abnormally high atlas height. (#618)
- Fixed support for any values of TexGlyphPadding (not just only 1). (#618)
- Atlas width is now properly based on total surface rather than glyph count (unless overridden with TexDesiredWidth). (#618)
- Fixed atlas builder so missing glyphs won't influence the atlas texture width. (#2233, #618)
- Fixed atlas builder so duplicate glyphs (when merging fonts) won't be included in the rasterized atlas. (#618)

ImFontAtlas: Rewrote stb_truetype based builder. ...

- Atlas width is now properly based on total surface rather than glyph count (unless overridden with TexDesiredWidth).
- Fixed atlas builder so missing glyphs won't influence the atlas texture width. (#2233)
- Fixed atlas builder so duplicate glyphs (when merging fonts) won't be included in the rasterized atlas.

Docking: Fixed docking a split node into the empty central node of a dockspace leading to the central node tag being incorrectly carried along. (#2109)

Internals: Added ImBoolVector helper.

imgui_freetype: Minor tweaks and comments.

ImVector: Added size_in_bytes() helper.