vo_opengl: always check GL errors in first 5 frames

Normally, we don't want to call glGetError() all the time, because this
supposedly causes slowdowns. (I could not measure any on Linux with
nVidia binary drivers; maybe it's due to the fact that we have only a
few, expensive calls per frame.)

However, having to ask users to add the "debug" suboption when trying to
diagnose problems is very annoying. Since most errors happen during
initialization only, enabling it for the first 5 frames only is an
interesting compromise.

libvo/vo_opengl.c

@@ -234,6 +234,8 @@ struct gl_priv {
     struct vo_rect dst_rect;    // video rectangle on output window
     int border_x, border_y;     // OSD borders
     int vp_x, vp_y, vp_w, vp_h; // GL viewport
+
+    int frames_rendered;
 };
 
 struct fmt_entry {
@@ -275,7 +277,7 @@ static void default_tex_params(struct GL *gl, GLenum target, GLint filter)
 
 static void debug_check_gl(struct gl_priv *p, const char *msg)
 {
-    if (p->use_gl_debug)
+    if (p->use_gl_debug || p->frames_rendered < 5)
         glCheckError(p->gl, msg);
 }
 
@@ -1238,6 +1240,8 @@ static void flip_page(struct vo *vo)
     {
         gl->Clear(GL_COLOR_BUFFER_BIT);
     }
+
+    p->frames_rendered++;
 }
 
 static int draw_slice(struct vo *vo, uint8_t *src[], int stride[], int w, int h,