ImFontAtlas; Re-arranging code to simplify implementation of imgui_freetype (#618)

ImFontAtlas: Draft of an api to submit custom rectangle (not exposed). Atlas default texture chunk using it. (WIP: we are still storing mouse UV outside in GImGui)

ImFontAtlas: Some shallow renaming + added an assert for clarification

Demo: Tweaked Fonts section.

ImVector: Added a const

ImFontAtlas: Shuffling some code inside Build() to make upcoming diffs less confusing (nb: we might break compat with forks of Build() like #618)

Merge pull request #1283 from onqtam/imgui_api ...

Added missing IMGUI_API to ImGuiTextFilter methods implemented in imgui.cpp

added missing IMGUI_API to ImGuiTextFilter methods implemented in imgui.cpp

Marked the weird IMGUI_ONCE_UPON_A_FRAME helper macro as obsolete. prefer using the more explicit ImGuiOnceUponAFrame. Removed the broken __LINE__ from IMGUI_ONCE_UPON_A_FRAME

OpenPopupEx() internal tweaks to receive an ImGuiID, BeginPopupContextXXX shortening unnecessarily long identifier.

PushID()/PopID() to not need to mark parent window as Accessed (needlessly waking up the root "Debug" window) (#747)

(api breaking) changed parameter order for BeginPopupContextWindow(), note that most uses relied on default parameters completely.

Comments (#402)

Merge branch 'mkeeter-is-popup-open'

Undo part of 32dbe83 to keep the same name for both overloads (#891, #799)

Merge branch 'is-popup-open' of https://github.com/mkeeter/imgui into mkeeter-is-popup-open ...

# Conflicts:
#	imgui.cpp

BeginPopupEx() uses ImGuiID internally

TODO list update, comments

Merge pull request #1282 from jadwallis/master ...

ImFontAtlas: Make font atlas padding between packed glyphs configurable

Whitespace fix to previous

Make font atlas packing padding configurable

TODO list update

Renamed

TODO list update (merging a first batch of things I had laying around, +60 entries)

ImFont::CalcWordWrapPositionA: minor optimization

PlotHistogram: bars are drawn based on the position of zero (#828)

Revert d2c219d #826 + tidying up ImFont::RenderChar()

Comments (#1034)

Moved TODO list to TODO.txt

Merge pull request #826 from swetland/patch ...

ImFont: add RenderGlyph() to allow rendering of individual glyphs post-query