feat(render): Renderer with LOD selection, 3.3 and 4.3+ upload paths, headless smoke test

include/plot/render/vertex_array.hpp

@@ -12,16 +12,29 @@ namespace plot::render {
 /// location 0 and `a_p1` at location 1, two floats each, advancing once per
 /// instance. A caller never writes a location, stride or offset; it stages a
 /// `LineBatch` and uploads its `data()`.
+///
+/// The instance buffer may sit behind either transfer strategy (see
+/// `UploadMode`). Only the mapped one moves the instances between slots, so
+/// `upload()` re-points the attributes when `InstanceStore::epoch()` changes and
+/// otherwise leaves them alone; the caller never has to know which path is live.
 class LineVertexArray {
  public:
   /// Creates the vertex array, the instance buffer and the attribute layout.
-  bool create(Status* st = nullptr) noexcept;
+  /// `mode` selects the instance buffer's transfer strategy and defaults to the
+  /// GL 3.3 baseline; a caller asking for `UploadMode::PersistentMapped` must
+  /// have checked the context can do it (see `Renderer::init()`).
+  bool create(Status* st = nullptr, UploadMode mode = UploadMode::Orphan) noexcept;
 
   /// Uploads `segments` instances from interleaved `x0,y0,x1,y1` floats and
   /// records how many the next draw should rasterise.
   void upload(const f32* interleaved, u32 segments) noexcept;
 
   void bind() const noexcept { vao_.bind(); }
+  /// Records that the draw just issued read the instance buffer's current slot,
+  /// which is what lets the mapped path reuse it safely. A no-op on the baseline
+  /// path, so a caller always calls it the same way.
+  void mark_drawn() noexcept { buffer_.mark_drawn(); }
+
   bool valid() const noexcept { return vao_.id() != 0; }
   u32 instance_count() const noexcept { return instances_; }
 
@@ -29,6 +42,8 @@ class LineVertexArray {
   LineBuffer buffer_;
   gl::VertexArray vao_;
   u32 instances_ = 0;
+  /// Instance-buffer generation the attribute pointers currently point into.
+  u32 epoch_ = 0;
 };
 
 /// Vertex array plus instance buffer of the text pipeline: `a_pos` at location 0
@@ -36,12 +51,14 @@ class LineVertexArray {
 /// advancing once per instance.
 class TextVertexArray {
  public:
-  bool create(Status* st = nullptr) noexcept;
+  bool create(Status* st = nullptr, UploadMode mode = UploadMode::Orphan) noexcept;
 
   /// Uploads `glyphs` instances from interleaved `x,y,u,v` floats.
   void upload(const f32* interleaved, u32 glyphs) noexcept;
 
   void bind() const noexcept { vao_.bind(); }
+  void mark_drawn() noexcept { buffer_.mark_drawn(); }
+
   bool valid() const noexcept { return vao_.id() != 0; }
   u32 instance_count() const noexcept { return instances_; }
 
@@ -49,6 +66,7 @@ class TextVertexArray {
   TextBuffer buffer_;
   gl::VertexArray vao_;
   u32 instances_ = 0;
+  u32 epoch_ = 0;
 };
 
 }  // namespace plot::render