ImGuiIntegration: document large mesh support

doc/namespaces.dox

@@ -365,6 +365,8 @@ See also @ref building-integration and @ref cmake-integration. See the
 
 Conversion of math classes and rendering of Dear ImGui using Magnum.
 
+@section ImGuiIntegration-usage Usage
+
 This library depends on [Dear ImGui](https://github.com/ocornut/imgui) and is
 built if `WITH_IMGUI` is enabled when building Magnum Integration. To use this
 library with CMake, put [FindMagnumIntegration.cmake](https://github.com/mosra/magnum-integration/blob/master/modules/FindMagnumIntegration.cmake)
@@ -390,6 +392,8 @@ set(WITH_IMGUI ON CACHE BOOL "" FORCE)
 add_subdirectory(magnum-integration EXCLUDE_FROM_ALL)
 @endcode
 
+@section ImGuiIntegration-configuration Configuration
+
 Dear ImGui configuration can normally be manipulated at compile time by
 defining `IMGUI_USER_CONFIG` which is then included by the Dear ImGui library.
 However, this library already uses this feature and defining

src/Magnum/ImGuiIntegration/Context.h

@@ -251,6 +251,35 @@ If you don't do that, the fonts stay at the original scale, not matching the
 new UI scaling anymore. If you didn't supply any custom font, the function will
 reconfigure the builtin font automatically.
 
+@section ImGuiIntegration-Context-large-meshes Large meshes
+
+Complex user interfaces or widgets like [ImPlot](https://github.com/epezent/implot)
+may end up creating large meshes with more than 64,000 vertices. Because ImGui
+defaults to 16-bit index buffers this can lead to asserts or visual errors.
+
+If the underlying GL context supports @ref GL::Mesh::setBaseVertex() "setting the base vertex for indexed meshes"
+the rendering backend sets the @cpp ImGuiBackendFlags_RendererHasVtxOffset @ce
+flag. This lets ImGui know the backend can handle per-draw vertex offsets,
+allowing meshes with a little over 2 million vertices. Support for that
+requires one of the following:
+
+-   OpenGL 3.2 and higher or extension `ARB_draw_elements_base_vertex`
+-   OpenGL ES 3.2 or extension `OES_draw_elements_base_vertex` or
+    `EXT_draw_elements_base_vertex`
+-   WebGL 2.0 and extension `WEBGL_draw_instanced_base_vertex_base_instance`
+
+If you can't guarantee that the required GL versions or extensions will be
+available at runtime (mostly relevant on WebGL) the next best option is to
+change ImGui's index type to 32-bit by adding the following line to the
+@ref ImGuiIntegration-configuration "ImGui user config":
+
+@code{.cpp}
+#define ImDrawIdx unsigned int
+@endcode
+
+This doubles the size of the index buffer but is guaranteed to work on all GL
+versions.
+
 @section ImGuiIntegration-Context-multiple-contexts Multiple contexts
 
 Each instance of @ref Context creates a new ImGui context. You can also pass an