osx: always deactivate the early opengl flush on macOS

early flushing only caused problems on macOS, which includes:
- performance problems and huge amount of dropped frames
- problems with playing back video files with fps close to the display
refresh rate
- rendering at twice the rate of the video fps
- not properly detected display refresh rate

we always deactivate any early flush for macOS to fix these problems.

DOCS/man/options.rst

@@ -5258,6 +5258,9 @@ The following video options are currently all specific to ``--vo=gpu`` and
     flipping GL front and backbuffers immediately (i.e. it doesn't call it
     in display-sync mode).
 
+    On OSX this is always deactivated because it only causes performance
+    problems and other regressions.
+
 ``--gpu-dumb-mode=<yes|no|auto>``
     This mode is extremely restricted, and will disable most extended
     features. That includes high quality scalers and custom shaders!

video/out/opengl/context_cocoa.c

@@ -36,6 +36,7 @@ const struct m_sub_options cocoa_conf = {
 
 struct priv {
     GL gl;
+    void (GLAPIENTRY *Flush)(void);
     CGLPixelFormatObj pix;
     CGLContextObj ctx;
 
@@ -49,6 +50,8 @@ static int set_swap_interval(int enabled)
     return (err == kCGLNoError) ? 0 : -1;
 }
 
+static void glFlushDummy(void) { }
+
 static void *cocoa_glgetaddr(const char *s)
 {
     void *ret = NULL;
@@ -138,6 +141,8 @@ static bool create_gl_context(struct ra_ctx *ctx)
 
     mpgl_load_functions(gl, (void *)cocoa_glgetaddr, NULL, ctx->vo->log);
     gl->SwapInterval = set_swap_interval;
+    p->Flush = gl->Flush;
+    gl->Flush = glFlushDummy;
 
     CGLReleasePixelFormat(p->pix);
 
@@ -155,9 +160,8 @@ static void cocoa_uninit(struct ra_ctx *ctx)
 static void cocoa_swap_buffers(struct ra_ctx *ctx)
 {
     struct priv *p = ctx->priv;
-    GL *gl = &p->gl;
     vo_cocoa_swap_buffers(ctx->vo);
-    gl->Flush();
+    p->Flush();
 }
 
 static bool cocoa_init(struct ra_ctx *ctx)