Allow glad to be excluded in favor of a custom frontend's own OpenGL function loader

[JesseTG]

Since melonDS DS is backed by libretro, the libretro frontend will have its own method of loading OpenGL functions. Using glad would conflict with that method, so I made it optional and added some instructions on using a private header.

This makes no difference to regular melonDS builds.

[CasualPokePlayer]

Why not just use gladLoadGLLoader (in which you pass a function pointer which can return gl functions by name).

[JesseTG]

I can do that? Huh. I'll be damned. Thanks for the tip! I'll give it a shot if I ever discover any problems with my weird workaround. If that doesn't work, then I'll revisit the issue.

[JesseTG]

Ah, I remember what the problem was; if I don't apply my compiler-flags workaround, then I get a cascade of compiler errors that look like this:

C:\tools\msys64\mingw64\bin\c++.exe -DCORE -DENABLE_JIT -DENABLE_OGLRENDERER -DHAVE_DYLIB -DHAVE_DYNAMIC -DHAVE_GETADDRINFO -DHAVE_JIT -DHAVE_JIT_FASTMEM -DHAVE_NETWORKING -DHAVE_NETWORKING_DIRECT_MODE -DHAVE_OPENGL -DHAVE_THREADS -DHAVE_ZLIB -DJIT_ENABLED -DMELONDSDS_NAME="\"melonDS DS\"" -DMELONDSDS_VERSION=\"0.2.0\" -DOGLRENDERER_ENABLED -Dlibretro_EXPORTS -IC:/Users/Jesse/Projects/melonds-ds/SYSTEM -IC:/Users/Jesse/Projects/melonds-ds/cmake-build-debug-windows/src/libretro -isystem C:/Users/Jesse/Projects/melonds-ds/cmake-build-debug-windows/_deps/libretro-common-src/include -isystem C:/Users/Jesse/Projects/melonds-ds/cmake-build-debug-windows/_deps/melonds-src/src -isystem C:/Users/Jesse/Projects/melonds-ds/cmake-build-debug-windows/_deps/glm-src -isystem C:/Users/Jesse/Projects/melonds-ds/cmake-build-debug-windows/_deps/melonds-src/src/frontend/qt_sdl -isystem C:/Users/Jesse/Projects/melonds-ds/src/libretro -isystem C:/tools/msys64/mingw64/bin/../include/slirp -isystem C:/tools/msys64/mingw64/bin/../include/glib-2.0 -isystem C:/tools/msys64/mingw64/bin/../lib/glib-2.0/include -DGLM_FORCE_XYZW_ONLY -g -std=gnu++17 -fdiagnostics-color=always -fPIC -pie -MD -MT src/libretro/CMakeFiles/libretro.dir/libretro.cpp.obj -MF src\libretro\CMakeFiles\libretro.dir\libretro.cpp.obj.d -o src/libretro/CMakeFiles/libretro.dir/libretro.cpp.obj -c C:/Users/Jesse/Projects/melonds-ds/src/libretro/libretro.cpp
In file included from C:/Users/Jesse/Projects/melonds-ds/cmake-build-debug-windows/_deps/melonds-src/src/PlatformOGL.h:7,
                 from C:/Users/Jesse/Projects/melonds-ds/cmake-build-debug-windows/_deps/melonds-src/src/OpenGLSupport.h:26,
                 from C:/Users/Jesse/Projects/melonds-ds/cmake-build-debug-windows/_deps/melonds-src/src/GPU_OpenGL.h:21,
                 from C:/Users/Jesse/Projects/melonds-ds/cmake-build-debug-windows/_deps/melonds-src/src/GPU.h:28,
                 from C:/Users/Jesse/Projects/melonds-ds/src/libretro/libretro.cpp:39:
C:/Users/Jesse/Projects/melonds-ds/cmake-build-debug-windows/_deps/melonds-src/src/frontend/glad/glad.h:27:2: error: #error OpenGL header already included, remove this include, glad already provides it
   27 | #error OpenGL header already included, remove this include, glad already provides it
      |  ^~~~~

Libretro #includes its own OpenGL function/macro definitions. melonDS also #includes its own OpenGL function/macro definitions (via glad).

[JesseTG]

Superseded by #2102