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

src/render/vertex_array.cpp

@@ -69,10 +69,15 @@ bool TextVertexArray::create(Status* st, UploadMode mode) noexcept {
   }
   vao_.bind();
   buffer_.bind();
+  // Three attributes now: the cell's corner, its atlas rect and the basis that
+  // rotates it (see TextInstance). All of them advance once per instance, like
+  // the line pipeline's, because the corners come from gl_VertexID.
   vao_.attrib(0u, 2, sizeof(TextInstance), offsetof(TextInstance, x));
   vao_.attrib(1u, 2, sizeof(TextInstance), offsetof(TextInstance, u));
+  vao_.attrib(2u, 2, sizeof(TextInstance), offsetof(TextInstance, ax));
   gl::glVertexAttribDivisor(0u, kPerInstance);
   gl::glVertexAttribDivisor(1u, kPerInstance);
+  gl::glVertexAttribDivisor(2u, kPerInstance);
   gl::glBindVertexArray(0);
   epoch_ = buffer_.epoch();
   return true;
@@ -90,6 +95,7 @@ void TextVertexArray::upload(const f32* interleaved, u32 glyphs) noexcept {
     buffer_.bind();
     vao_.attrib(0u, 2, sizeof(TextInstance), base + offsetof(TextInstance, x));
     vao_.attrib(1u, 2, sizeof(TextInstance), base + offsetof(TextInstance, u));
+    vao_.attrib(2u, 2, sizeof(TextInstance), base + offsetof(TextInstance, ax));
     epoch_ = buffer_.epoch();
   }
   instances_ = glyphs;