client API: clarify mpv_opengl_cb_render() viewport parameter

Basically, the idea behind the vp parameter is broken - I guess the
intention was to enable rendering to a specific subrectangle of the
target framebuffer, but there's nothing to specify the actual target
rectangle (the VO will still clear e.g. the borders between video and
framebuffer borders).

We're not going to keep the current semantics either with the upcoming
rework of vo_opengl, so declare this for broken. Maybe we can introduce
a function later which does this properly.

libmpv/opengl_cb.h

@@ -186,11 +186,14 @@ int mpv_opengl_cb_init_gl(mpv_opengl_cb_context *ctx, const char *exts,
  *            postprocessing, it will always bind and unbind FBOs itself. If
  *            you want mpv to render on the main framebuffer, pass 0.
  * @param vp Viewport to render on. The renderer will essentially call:
- *            glViewport(vp[0], vp[1], vp[2], vp[3]);
+ *            glViewport(0, 0, vp[2], vp[3]);
  *           before rendering. The height (vp[3]) can be negative to flip the
  *           image - the renderer will flip it before setting the viewport
  *           (typically you want to flip the image if you are rendering
  *           directly to the main framebuffer).
+ *           vp[0] and vp[1] should be set to 0.
+ *           The viewport width and height imply the target FBO or framebuffer's
+ *           size. It will always render to the full size of it.
  * @return the number of left frames in the internal queue to be rendered
  */
 int mpv_opengl_cb_render(mpv_opengl_cb_context *ctx, int fbo, int vp[4]);