refactor(render): TextBackend owns its atlas, and staged glyphs carry a rotation basis

include/plot/gl/handle.hpp

@@ -175,7 +175,8 @@ class Texture final : public Handle<Texture> {
   /// data (row padding is disabled, so any width is valid). It sets
   /// `GL_UNPACK_ALIGNMENT` to 1 to do so and leaves it there -- a caller that
   /// shares the context with other uploaders saves and restores it, as
-  /// `Renderer` does around its atlas and its frames.
+  /// `render::TextAtlas` does around every glyph-atlas upload, whenever in a
+  /// frame that upload happens to run.
   void upload_r8(u32 width, u32 height, const void* pixels) const noexcept;
 
   /// Binds to `unit` (e.g. `GL_TEXTURE0`).