feat(text): optional vendored stb_truetype backend for runtime external fonts

include/plot/render/renderer.hpp

@@ -77,6 +77,32 @@ class Renderer {
   /// `Unsupported`; anything the driver refuses is `GLFailure` plus its log.
   bool init(Status* st = nullptr) noexcept;
 
+  /// The face every frame this renderer draws uses: the embedded one until a host
+  /// installs another here.
+
+  /// `init()` binds `render::EmbeddedMonoFont`, and `draw()` installs the binding
+  /// on the plot it is given (replacing whatever a host set with
+  /// `Plot::set_text_backend()`, which is how a plot with no renderer keeps its
+  /// own), so this is the way to draw with a face of your own -- a
+  /// `plot::text::TrueTypeFont` from the optional target, say, or any other
+  /// `TextBackend`. Passing null restores the embedded face. The swap takes
+  /// effect on the next frame, which re-reads the backend's metrics at
+  /// `style().device(axis_font_px)` and re-uploads its atlas -- so nothing here
+  /// has to know whether the replacement is a different face, the same face
+  /// re-baked at another size, or a backend whose pixels changed: the revision
+  /// comparison in `render::TextAtlas` decides, and a replacement whose atlas is
+  /// already on the GPU under the same revision is uploaded anyway (see the
+  /// implementation for why that is the safe direction).
+  ///
+  /// The backend is borrowed, not owned: it must outlive the frames drawn with it,
+  /// which is the same lifetime rule `Plot::set_text_backend()` states.
+  void set_text_backend(const render::TextBackend* font) noexcept;
+
+  /// The backend the last frame was drawn with, or the embedded face when the
+  /// host installed none. The renderer's own wrapper while a frame is in flight,
+  /// so a caller reads this after `draw()` to see what the plot was handed.
+  const render::TextBackend* text_backend() const noexcept { return font_; }
+
   /// Starts a frame: records `fb_w` x `fb_h` for the projection, flips
   /// `viewport_px` (top-left origin, like every `Rect` in this library) into GL's
   /// bottom-left origin once, clears that region to `style().background` and
@@ -387,10 +413,14 @@ class Renderer {
   /// numbers.
   void draw_legend(const Plot& plot, const PlotLayout& layout);
 
-  /// The built-in backend the renderer owns. Every use of it goes through the
-  /// `TextBackend` interface -- `self`-typed nowhere -- so swapping it for
-  /// another face is a change to this member alone.
+  /// The built-in backend the renderer owns, and the face the frames draw with:
+  /// `set_text_backend()` writes `font_`, which starts here and comes back here
+  /// when a host passes null. Every use of the face goes through the
+  /// `TextBackend` interface -- `self`-typed nowhere -- so a host's backend and
+  /// this one are interchangeable, and the only thing that has to happen on a swap
+  /// is the atlas re-upload `set_text_backend()` arranges.
   render::EmbeddedMonoFont default_font_;
+  const render::TextBackend* font_ = &default_font_;
   AxisFont font_backend_;
   render::Style style_;