Foundation for a selectable hardware (OpenGL) renderer alongside the software
rasterizer. No behavior change: software remains the default and routes through
the facade to the exact same sw_* functions.
- config: [video] renderer = "software" (default) | "opengl" (config_loader).
- gpu_render.h/.c: gr_* facade + GpuRenderBackend vtable. gpu.c (50 call sites)
and main.cpp (present/scale/filter) now call gr_* instead of sw_* directly.
The software backend table points straight at sw_*, so the SW path is
byte-identical.
- gpu_gl_renderer.c: Phase-1 stub. gl_backend_get() returns NULL, so
requesting renderer="opengl" logs a notice and falls back to software.
- main.cpp: selects the backend (gr_set_backend) before gpu_init().
Phase 2 (next) builds the real GL backend: SDL GL context, VRAM-as-texture/FBO,
shader rasterization at internal resolution, present. Phase 3: coherency,
transparency/dither/mask, then default-with-fallback.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Matthew Stanley