Add GL error handling, resource fallbacks and GPU timing; speed up lighting

Bug fixes:
- Release static GL objects (UBOs, logo texture) before the context is
  destroyed via new Renderer::shutdown()
- Fix max point/spot light warnings never firing
- Show frame time in ms instead of seconds
- Size the viewport framebuffer in physical pixels on HiDPI displays
- Stop double-flipping OBJ UVs; add flip_uvs option to load_obj for
  models with a top-left UV origin (backpack)

Build:
- Track header dependencies (-MMD -MP) so header edits trigger rebuilds
- Create build/ automatically

Error handling:
- Add gldebug module: KHR_debug output routed to the logger in debug
  builds, object labels, and error/status string helpers
- Validate inputs and check GL errors when creating textures, meshes,
  framebuffers, uniform buffers and shaders; FrameBuffer::resize() and
  Renderer::init() now report failure

Resources:
- Add ResourceCache: shares textures and shaders by path and substitutes
  fallbacks (magenta checkerboard texture, unlit shader, cube mesh) for
  assets that fail to load; specular maps are optional
- load_obj never fails, a broken model becomes a fallback cube
- Renderer draws submeshes with broken materials using the fallback
  material instead of skipping them
- Add Texture::create_from_pixels, Shader::create_from_source and
  Mesh::create_cube
- Cache uniform locations in Shader, looked up with string_view so
  hits don't allocate; missing uniforms warn once

Performance:
- Add CpuTimer and GpuTimer (GL_TIME_ELAPSED, non-blocking readback)
  and a Performance section in the UI with averaged overlay numbers
- Sample material textures once per fragment and apply material colors
  after accumulating lights
- Pass camera position through CameraBlock instead of inverting the
  view matrix per fragment
- Compute the normal matrix on the CPU instead of per vertex
- Skip spot light work for fragments outside the outer cone

src/mesh.hpp

@@ -14,6 +14,8 @@ namespace Fan
 class Mesh
 {
 public:
+    // A range drawn with one material. Offset and count are in indices, or in
+    // vertices if the mesh has no index buffer
     struct SubMesh
     {
         GLuint index_offset{};
@@ -21,9 +23,13 @@ public:
         Rc<Material> material{};
     };
 
+    // Without indices the vertices are drawn in order. Without submeshes a single
+    // one with no material covers the whole mesh
     static Rc<Mesh> create(const std::vector<Vertex>& vertices,
                            const std::vector<GLuint>& indices = {},
                            const std::vector<SubMesh>& submeshes = {});
+    // Unit cube centered at the origin, with normals and texture coordinates
+    static Rc<Mesh> create_cube();
     Mesh(const std::vector<Vertex>& vertices,
          const std::vector<GLuint>& indices,
          const std::vector<SubMesh>& submeshes,
@@ -45,6 +51,7 @@ public:
 
     const std::vector<SubMesh>& get_submeshes() const;
 
+    // Geometry only: the caller binds the shader and material first
     void draw() const;
     void draw_submesh(GLuint index_offset, GLuint index_count) const;