feat(api): make FreeType a toggleable option which is disabled by default (#254)

* deps(build): bump FreeType from 2.12.1 to 2.13.3

* fix(generator): add missing ImGuiFreeTypeBuilderFlags

* feat(generator): add possibility to provide additional defines specifically for AST

* feat(api): make FreeType optional and disabled by default

FreeType renderer is still included in the final build.
Its usage is toggleable and DISABLED by default.

build.gradle

@@ -54,6 +54,7 @@ tasks.register('generateAst', GenerateAst) {
     headerFiles = [
             file('include/imgui/imgui.h'),
             file('include/imgui/imgui_internal.h'),
+            file('include/imgui/misc/freetype/imgui_freetype.h'),
             file('include/ImGuiFileDialog/ImGuiFileDialog.h'),
             file('include/imgui-knobs/imgui-knobs.h'),
             file('include/imguizmo/ImGuizmo.h'),