Updated to stb_truetype 1.19 (include minor fix for #1622)

Examples: SDL: Added Changelog at the top of the imgui_impl files. (#1618)

ImFont: stb_truetype now rounding Ascent/Descent the same way as freetype does, they seem to vertically align better this way.

ImFont: DisplayOffset.y defaults to 0 instead of +1.

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Comments, moved ImFontAtlas::Flags to "public" area.

ImFontAtlas: Added ImFontAtlasFlags_NoPowerOfTwoHeight, ImFontAtlasFlags_NoMouseCursors flags. (#1613)

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Travis: testing without adding external apt repositories (which frequently fails), may be unnecessary today

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ImFontAtlas: if stbtt_PackBegin() because of failing allocation we return a little more nicely. (keeps Coverity static analyzer happy!)

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Demo: Fonts; Added Font Offset.

Fixed legacy forward function IsRootWindowOrAnyChildHovered() broken by a63fbbc

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Added sample gif, removed section about performances (unnecessary)

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Fixed warning: variable ‘L1’ set but not used [-Wunused-but-set-variable]

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Contributing guidelines for issues