vo: make opengl-cb first in the autoprobing order

This should be helpful for the new OSX Cocoa backend, which uses
opengl-cb internally. Since it comes with a behavior change that could
possibly interfere with libmpv/opengl_cb users, we mark it as explicit
API change.

DOCS/client-api-changes.rst

@@ -32,6 +32,12 @@ API changes
 
 ::
 
+ --- mpv 0.29.0 ---
+ 1.27   - make opengl-cb the default VO. This causes a subtle behavior change
+          if the API user called mpv_opengl_cb_init_gl(), but does not set
+          the "vo" option. Before, it would still have used another VO (like
+          on the CLI, e.g. vo=gpu). Now it'll behave as if vo=opengl-cb was
+          used.
  --- mpv 0.28.0 ---
  1.26   - remove glMPGetNativeDisplay("drm") support
         - add mpv_opengl_cb_window_pos and mpv_opengl_cb_drm_params and

libmpv/client.h

@@ -209,7 +209,7 @@ extern "C" {
  * relational operators (<, >, <=, >=).
  */
 #define MPV_MAKE_VERSION(major, minor) (((major) << 16) | (minor) | 0UL)
-#define MPV_CLIENT_API_VERSION MPV_MAKE_VERSION(1, 26)
+#define MPV_CLIENT_API_VERSION MPV_MAKE_VERSION(1, 27)
 
 /**
  * The API user is allowed to "#define MPV_ENABLE_DEPRECATED 0" before

video/out/vo.c

@@ -66,6 +66,9 @@ extern const struct vo_driver video_out_tct;
 
 const struct vo_driver *const video_out_drivers[] =
 {
+#if HAVE_GL
+    &video_out_opengl_cb,
+#endif
 #if HAVE_ANDROID
     &video_out_mediacodec_embed,
 #endif
@@ -103,9 +106,6 @@ const struct vo_driver *const video_out_drivers[] =
 #endif
 #if HAVE_ENCODING
     &video_out_lavc,
-#endif
-#if HAVE_GL
-    &video_out_opengl_cb,
 #endif
     NULL
 };

video/out/vo_opengl_cb.c

@@ -554,6 +554,7 @@ static int preinit(struct vo *vo)
     struct vo_priv *p = vo->priv;
     p->ctx = vo->extra.opengl_cb_context;
     if (!p->ctx) {
+        if (!vo->probing)
             MP_FATAL(vo, "No context set.\n");
         return -1;
     }