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

CMakeLists.txt

@@ -14,6 +14,8 @@ add_library(plot_core
   src/gl/loader.cpp
   src/gl/handle.cpp
   src/render/shaders.cpp
+  src/render/text.cpp
+  src/render/text_atlas.cpp
   src/render/bitmap_font.cpp
   src/render/primitives.cpp
   src/render/buffer.cpp