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.).
portersky
5ec8cfc735