Add Libretro Slang Shader / Filters Support (Frontend UI)

[Pedro-HDSR]

melonDS — Shader/Filter UI Extension

This PR adds a runtime shader filter system to melonDS, allowing users to apply visual filters (scalers, CRT effects, anti-aliasing, handheld LCD grids, etc.) directly from the View → Filters window without needing to edit config files manually.

Key Implementations:

• 35 curated shader presets from the libretro slang-shaders collection, including xBRZ, HQx, Scale2x, Super-xBR, SMAA, FXAA, CRT EasyMode/Geom/Hyllian/Lottes, GameBoy DMG, LCD Grid, NTSC composite, and more
• Filter stacking — combine multiple shaders (e.g. xBRZ upscale + CRT scanlines)
• Adjustable parameters — sliders for every shader parameter with extended ranges for experimentation
• Persistent settings — your filter selection and parameter tweaks are saved across sessions
• No External Dependencies Required for End-Users: Shaders are compiled at runtime via glslang and spirv-cross.

The slang-shaders submodule remains pristine — zero modifications to any shader source. All integration is in the C++ frontend layer.

Architectural Notes & CI/CD Changes:

• I kept modifications to the original core code to an absolute minimum. The major architectural addition is the inclusion of the glslang and spirv-cross libraries. These are the official Khronos compilers and the standard, high-performance method for reading and translating Slang shaders natively.
• CI/CD and Packaging: To ensure the res/slang-shaders directory is properly distributed to end-users out-of-the-box, I updated the GitHub Actions workflows. The pipeline now packages the executable alongside the res/ folder (bundled into a .zip for Windows/BSD, and natively injected into the .AppImage / .app directories for Linux/macOS).

Modified files (focused on GLSLANG/SPIRV-CROSS integration and packaging):

vcpkg.json
.github/workflows/build-bsd.yml
.github/workflows/build-macos.yml
.github/workflows/build-ubuntu.yml
.github/workflows/build-windows.yml
CMakeLists.txt
src/frontend/qt_sdl/CMakeLists.txt
src/frontend/qt_sdl/Config.cpp
src/frontend/qt_sdl/Screen.cpp
src/frontend/qt_sdl/Screen.h
src/frontend/qt_sdl/Window.cpp
src/frontend/qt_sdl/Window.h

Added files (Core shader implementation and UI):

.gitmodules
res/slang-shaders/
src/frontend/qt_sdl/FrontendShader.h
src/frontend/qt_sdl/ShaderConfigDialog.cpp
src/frontend/qt_sdl/ShaderConfigDialog.h
src/frontend/qt_sdl/ShaderManager.cpp
src/frontend/qt_sdl/ShaderManager.h
src/frontend/qt_sdl/ShaderParser.cpp
src/frontend/qt_sdl/ShaderParser.h
src/frontend/qt_sdl/SlangPreset.cpp
src/frontend/qt_sdl/SlangPreset.h

Showcase:

[v-fox]

This is a gross violation of https://en.wikipedia.org/wiki/Filesystem_Hierarchy_Standard
Anything related to this "./res" directory should be redone and put into appropriate places.