Add shader presets (CRT/Scanlines) and a functional screenshot hotkey

[ClaudioVarandas]

Summary

Adds GLSL shader presets (CRT + Scanlines, 3 strength tiers each) to the
existing but unused shader preset loader, and a functional player-facing
screenshot hotkey (F12). Also fixes the launcher round-trip that was
silently dropping the selected shader instead of persisting/applying it.

What changed

• Shader presets: assets/shaders/{crt,scanlines}{,-light,-strong}.glslp
  for src/glsl_shader.c's existing RetroArch-style preset loader, which
  had zero presets shipped. GameInfo.has_shader = 1 turns on the
  launcher's Settings → Video → Shader dropdown (already implemented in
  the shared launcher, just never enabled for this game).
• Shader persistence fix: the launcher's selection now round-trips
  through RecompLauncherCSettings.shader_path into g_config.shader
  and gets written back via WriteConfigFile's new OutputMethod/
  Shader keys — previously the picker showed a choice but it never
  reached config.ini or the renderer.
• Screenshot hotkey: new Screenshot command (F12 default,
  OpenGL backend only — same limitation the shader path already has).
  Captures the fully composited framebuffer via glReadPixels (custom
  widescreen renderer + any active shader included), writes a timestamped
  BMP to screenshots/, with a 0.5s debounce against SDL key auto-repeat
  and a brief time-based flash for feedback in fullscreen.
• Build robustness: CMakeLists.txt gains a shader-asset staging
  target that removes+recopies assets/shaders/ on every build (a plain
  POST_BUILD copy only ran when the executable itself relinked, leaving
  stale files behind after a rename/delete).

Testing

• ctest 5/5 on both Debug and Release builds.
• Manual: shader selection persists across relaunch and renders visually;
  F12 saves a valid BMP and shows the flash; verified with a diagnostic
  solid-color shader that the full launcher → config.ini → OpenGL
  pipeline works end to end.
• Investigated a widescreen-looks-4:3 report during testing: traced it to
  the attract demo's game_state (30) falling outside
  sm_renderer.c's room_state() allowlist — a pre-existing, documented
  renderer limitation for the demo/menu path, unrelated to this change.
  Confirmed widescreen renders correctly in real gameplay.

Notes

• No snesrecomp or recomp-ui (submodule) changes — everything is
  scoped to this repo.
• Raw .glsl shader sources live under assets/shaders/.src/ (dot-, not
  underscore-, prefixed) so the launcher's picker treats them as private
  without colliding with this repo's _* scratch-file .gitignore rule.

Note: this code was made with AI assistance, tested and validated by me.