feat: add gfx pipeline abstraction with render targets

Add cbt/gfx layer (pipeline + render_target) inspired by Sokol but
C++-friendly with RAII and pipeline_desc. Supports building full
graphics pipelines (VS/IA/raster/PS/OM), render-to-texture (FBO),
and post-processing steps (scene -> RT -> fullscreen quad with
sampler + vignette).

- Depth/state/viewport/texture cleanup for reliable scene switching.
- Updated cube/sphere to demonstrate (sphere uses RT+post).
- Vulkan backend easy via PIMPL in impl (same public API).
- Fixed depth test, viewport restore, and state leakage on switch.

Followed coding conventions (snake_case, trailing returns, east const,
include order, no ; after ns/class, etc.).

scenes/cube.hpp

@@ -5,9 +5,7 @@
 #include "glm/glm.hpp"
 
 #include "cbt/scene.hpp"
-#include "cbt/opengl/buffer.hpp"
-#include "cbt/opengl/shader.hpp"
-#include "cbt/opengl/vao.hpp"
+#include "cbt/gfx.hpp"
 
 namespace cbt::scenes {
 
@@ -19,18 +17,11 @@ public:
     auto render(int width, int height) -> void override;
 
 private:
-    opengl::shader m_prog;
-    opengl::buffer m_vbo;
-    opengl::vao m_vao;
-
-    GLint m_loc_proj = -1;
-    GLint m_loc_view = -1;
-    GLint m_loc_model = -1;
+    gfx::pipeline m_pipeline;
 
     std::chrono::steady_clock::time_point m_start;
 
-    auto build_mesh() -> void;
-    auto build_shader() -> bool;
+    auto build_pipeline() -> bool;
 };
 
 }