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

include/plot/render/program.hpp

@@ -48,8 +48,14 @@ class TextProgram {
 
   /// `u_viewport`: render target size in pixels.
   void set_viewport(Vec2 px) const noexcept;
-  /// `u_px_range`: pixels per glyph cell, i.e. the on-screen size of one glyph.
+  /// `u_px_range`: the glyph cell's size on screen, in pixels (width and
+  /// height -- a cell need not be square).
   void set_px_range(Vec2 px) const noexcept;
+  /// `u_glyph_uv`: the glyph cell's size in the atlas, normalised. It is the
+  /// rect every staged glyph is read through, so it is one size per draw: a
+  /// batch stages glyphs from one atlas, and `TextBatch::glyph_cell_uv()` is
+  /// where it comes from.
+  void set_glyph_uv(Vec2 uv) const noexcept;
   /// `u_tex_size`: atlas size in texels.
   void set_texture_size(Vec2 texels) const noexcept;
   /// `u_color`: tint colour, alpha included.