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

Makefile

@@ -2,6 +2,7 @@ CXX        := clang++
 CXXFLAGS   := -Wall -Wextra -std=c++17
 CC         := clang
 CFLAGS     := -Wall -Wextra
+DEPFLAGS   := -MMD -MP
 
 INCLUDES   := -I./src/ \
               -I./vendor/glad/include/ \
@@ -17,14 +18,17 @@ TARGET     := build/fan
 
 OBJS       := build/camera.o \
               build/framebuffer.o \
+              build/gldebug.o \
               build/loader.o \
               build/main.o \
               build/material.o \
               build/mesh.o \
               build/renderer.o \
+              build/resourcecache.o \
               build/scene.o \
               build/shader.o \
               build/texture.o \
+              build/timer.o \
               build/transform.o \
               build/uniformbuffer.o \
               build/utils.o \
@@ -33,30 +37,31 @@ OBJS       := build/camera.o \
               build/stb_image.o \
               build/tiny_obj_loader.o \
 
-IMGUI_OBJS := build/imgui.o build/imgui_demo.o build/imgui_draw.o build/imgui_tables.o build/imgui_widgets.o \
+IMGUI_OBJS := build/imgui.o build/imgui_demo.o build/imgui_draw.o \
+              build/imgui_tables.o build/imgui_widgets.o \
               build/imgui_impl_glfw.o build/imgui_impl_opengl3.o
 
 .PHONY: all clean
 
 all: $(TARGET)
 
-$(TARGET): $(OBJS) $(IMGUI_OBJS)
+$(TARGET): $(OBJS) $(IMGUI_OBJS) | build
 	$(CXX) $(CXXFLAGS) -o $@ $^ $(LDFLAGS)
 
-build/%.o: src/%.cpp
-	$(CXX) $(CXXFLAGS) $(INCLUDES) -c $< -o $@
+build/%.o: src/%.cpp | build
+	$(CXX) $(CXXFLAGS) $(DEPFLAGS) $(INCLUDES) -c $< -o $@
 
-build/glad.o: vendor/glad/src/gl.c
+build/glad.o: vendor/glad/src/gl.c | build
 	$(CC) $(CFLAGS) -I./vendor/glad/include/ -c $< -o $@
 
-build/stb_image.o: vendor/stb/stb_image.h
+build/stb_image.o: vendor/stb/stb_image.h | build
 	$(CC) $(CFLAGS) -DSTB_IMAGE_IMPLEMENTATION -xc -c $< -o $@
 
-build/tiny_obj_loader.o: vendor/tinyobjloader/tiny_obj_loader.h
+build/tiny_obj_loader.o: vendor/tinyobjloader/tiny_obj_loader.h | build
 	$(CXX) $(CXXFLAGS) -DTINYOBJLOADER_IMPLEMENTATION -xc++ -c $< -o $@
 
 define imgui_rule
-build/$(1): vendor/imgui/$(2)
+build/$(1): vendor/imgui/$(2) | build
 	$(CXX) $(CXXFLAGS) -I./vendor/imgui/ -c $$< -o $$@
 endef
 
@@ -68,5 +73,10 @@ $(eval $(call imgui_rule,imgui_widgets.o,imgui_widgets.cpp))
 $(eval $(call imgui_rule,imgui_impl_glfw.o,backends/imgui_impl_glfw.cpp))
 $(eval $(call imgui_rule,imgui_impl_opengl3.o,backends/imgui_impl_opengl3.cpp))
 
+build:
+	mkdir -p build
+
 clean:
-	rm -f build/*.o $(TARGET)
+	rm -f build/*.o build/*.d $(TARGET)
+
+-include $(OBJS:.o=.d)