video: Rename SDL_GL_DeleteContext to SDL_GL_DestroyContext.

Turns out that there isn't a strong OpenGL naming convention for "Delete" ...
WGL offers "wglDeleteContext" but the GLX equivalent is "glxDestroyContext"
and then EGL sealed the deal by going with Destroy as well! Since it matches
SDL3 naming conventions (Create/Destroy), we're renaming it.

Fixes #10197.

test/testgl.c

@@ -63,7 +63,7 @@ static void quit(int rc)
 {
     if (context) {
         /* SDL_GL_MakeCurrent(0, NULL); */ /* doesn't do anything */
-        SDL_GL_DeleteContext(context);
+        SDL_GL_DestroyContext(context);
     }
     SDLTest_CommonQuit(state);
     /* Let 'main()' return normally */