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)

Author: ocornut

docs/CHANGELOG.txt

@@ -72,9 +72,11 @@ Other Changes:
   Missing calls to End(), past the assert, should not lead to crashes or to the fallback Debug window appearing on screen.
   Those changes makes it easier to integrate dear imgui with a scripting language allowing, given asserts are redirected
   into e.g. an error log and stopping the script execution.
-- ImFontAtlas: Stb: Atlas width is now properly based on total surface rather than glyph count (unless overridden with TexDesiredWidth).
-- ImFontAtlas: Stb: Fixed atlas builder so missing glyphs won't influence the atlas texture width. (#2233)
-- ImFontAtlas: Stb: Fixed atlas builder so duplicate glyphs (when merging fonts) won't be included in the rasterized atlas.
+- ImFontAtlas: Stb and FreeType: Atlas width is now properly based on total surface rather than glyph count (unless overridden with TexDesiredWidth).
+- ImFontAtlas: Stb and FreeType: Fixed atlas builder so missing glyphs won't influence the atlas texture width. (#2233)
+- ImFontAtlas: Stb and FreeType: Fixed atlas builder so duplicate glyphs (when merging fonts) won't be included in the rasterized atlas.
+- ImFontAtlas: FreeType: Fixed abnormally high atlas height.
+- ImFontAtlas: FreeType: Fixed support for any values of TexGlyphPadding (not just only 1).
 - ImDrawList: Optimized some of the functions for performance of debug builds where non-inline function call cost are non-negligible.
   (Our test UI scene on VS2015 Debug Win64 with /RTC1 went ~5.9 ms -> ~4.9 ms. In Release same scene stays at ~0.3 ms.)
 - IO: Added BackendPlatformUserData, BackendRendererUserData, BackendLanguageUserData void* for storage use by back-ends.

misc/freetype/README.md

@@ -1,14 +1,14 @@
 # imgui_freetype
 
-This is an attempt to replace stb_truetype (the default imgui's font rasterizer) with FreeType. 
-Currently not optimal and probably has some limitations or bugs.
-By [Vuhdo](https://github.com/Vuhdo) (Aleksei Skriabin). Improvements by @mikesart. Maintained by @ocornut.
+Build font atlases using FreeType instead of stb_truetype (the default imgui's font rasterizer).
+<br>by @vuhdo, @mikesart, @ocornut.
 
-**Usage**
-1. Get latest FreeType binaries or build yourself.
+### Usage
+
+1. Get latest FreeType binaries or build yourself (under Windows you may use vcpkg with `vcpkg install freetype`).
 2. Add imgui_freetype.h/cpp alongside your imgui sources.
 3. Include imgui_freetype.h after imgui.h.
-4. Call ImGuiFreeType::BuildFontAtlas() *BEFORE* calling ImFontAtlas::GetTexDataAsRGBA32() or ImFontAtlas::Build() (so normal Build() won't be called):
+4. Call `ImGuiFreeType::BuildFontAtlas()` *BEFORE* calling `ImFontAtlas::GetTexDataAsRGBA32()` or `ImFontAtlas::Build()` (so normal Build() won't be called):
 
 ```cpp
 // See ImGuiFreeType::RasterizationFlags
@@ -17,13 +17,14 @@ ImGuiFreeType::BuildFontAtlas(io.Fonts, flags);
 io.Fonts->GetTexDataAsRGBA32(&pixels, &width, &height);
 ```
 
-**Gamma Correct Blending**
+### Gamma Correct Blending
+
 FreeType assumes blending in linear space rather than gamma space.
 See FreeType note for [FT_Render_Glyph](https://www.freetype.org/freetype2/docs/reference/ft2-base_interface.html#FT_Render_Glyph).
 For correct results you need to be using sRGB and convert to linear space in the pixel shader output.
 The default imgui styles will be impacted by this change (alpha values will need tweaking).
 
-**Test code Usage**
+### Test code Usage
 ```cpp
 #include "misc/freetype/imgui_freetype.h"
 #include "misc/freetype/imgui_freetype.cpp"
@@ -42,16 +43,15 @@ while (true)
    if (freetype_test.UpdateRebuild())
    {
       // REUPLOAD FONT TEXTURE TO GPU
-      // e.g ImGui_ImplGlfwGL3_InvalidateDeviceObjects() + ImGui_ImplGlfwGL3_CreateDeviceObjects()
+      // e.g ImGui_ImplOpenGL3_DestroyDeviceObjects() + ImGui_ImplOpenGL3_CreateDeviceObjects()
    }
    ImGui::NewFrame();
    freetype_test.ShowFreetypeOptionsWindow();
    ...
-   }
 }
 ```
 
-**Test code**
+### Test code
 ```cpp
 #include "misc/freetype/imgui_freetype.h"
 #include "misc/freetype/imgui_freetype.cpp"
@@ -61,7 +61,7 @@ struct FreeTypeTest
     enum FontBuildMode
     {
         FontBuildMode_FreeType,
-        FontBuildMode_Stb,
+        FontBuildMode_Stb
     };
 
     FontBuildMode BuildMode;
@@ -120,14 +120,7 @@ struct FreeTypeTest
 };
 ```
 
-**Known issues**
-- Output texture has excessive resolution (lots of vertical waste).
+### Known issues
 - FreeType's memory allocator is not overridden.
 - `cfg.OversampleH`, `OversampleV` are ignored (but perhaps not so necessary with this rasterizer).
 
-**Obligatory comparison screenshots**
-
-Using Windows built-in segoeui.ttf font. Open in new browser tabs, view at 1080p+.
-
-![freetype rasterizer](https://raw.githubusercontent.com/wiki/ocornut/imgui_club/images/freetype_20170817.png)
-