Use SDL_GL_DeleteContext instead of Cocoa_GL_DeleteContext for context cleanup

That will make the current context NULL before deleting the context.

src/video/cocoa/SDL_cocoaopengl.m

@@ -380,8 +380,8 @@ Cocoa_GL_CreateContext(_THIS, SDL_Window * window)
     interval = 0;
     [context setValues:&interval forParameter:NSOpenGLCPSwapInterval];
 
-    if ( Cocoa_GL_MakeCurrent(_this, window, (__bridge SDL_GLContext)context) < 0 ) {
-        Cocoa_GL_DeleteContext(_this, (__bridge SDL_GLContext)context);
+    if (Cocoa_GL_MakeCurrent(_this, window, sdlcontext) < 0) {
+        SDL_GL_DeleteContext(sdlcontext);
         SDL_SetError("Failed making OpenGL context current");
         return NULL;
     }
@@ -395,27 +395,27 @@ Cocoa_GL_CreateContext(_THIS, SDL_Window * window)
 
         glGetStringFunc = (const GLubyte *(APIENTRY *)(GLenum)) SDL_GL_GetProcAddress("glGetString");
         if (!glGetStringFunc) {
-            Cocoa_GL_DeleteContext(_this, (__bridge SDL_GLContext)context);
+            SDL_GL_DeleteContext(sdlcontext);
             SDL_SetError ("Failed getting OpenGL glGetString entry point");
             return NULL;
         }
 
         glversion = (const char *)glGetStringFunc(GL_VERSION);
         if (glversion == NULL) {
-            Cocoa_GL_DeleteContext(_this, (__bridge SDL_GLContext)context);
+            SDL_GL_DeleteContext(sdlcontext);
             SDL_SetError ("Failed getting OpenGL context version");
             return NULL;
         }
 
         if (SDL_sscanf(glversion, "%d.%d", &glversion_major, &glversion_minor) != 2) {
-            Cocoa_GL_DeleteContext(_this, (__bridge SDL_GLContext)context);
+            SDL_GL_DeleteContext(sdlcontext);
             SDL_SetError ("Failed parsing OpenGL context version");
             return NULL;
         }
 
         if ((glversion_major < _this->gl_config.major_version) ||
            ((glversion_major == _this->gl_config.major_version) && (glversion_minor < _this->gl_config.minor_version))) {
-            Cocoa_GL_DeleteContext(_this, (__bridge SDL_GLContext)context);
+            SDL_GL_DeleteContext(sdlcontext);
             SDL_SetError ("Failed creating OpenGL context at version requested");
             return NULL;
         }