render: better integrate bind_buffer with begin/end

[emersion]

Right now, one needs to do the following to render:

wlr_egl_make_current(egl, EGL_NO_SURFACE); // done in the backend
wlr_renderer_bind_buffer(renderer, buffer); // done in the backend
wlr_renderer_begin(renderer, width, height); // done in the compositor

// Perform the rendering operations

wlr_renderer_end(renderer); // done in the compositor
wlr_renderer_bind_buffer(renderer, NULL); // done in the backend
wlr_egl_unset_current(egl); // done in the backend

Once #2615 (comment) is done, this could be simplified to:

wlr_renderer_bind_buffer(renderer, buffer);
wlr_renderer_begin(renderer, width, height);

// Perform the rendering operations

wlr_renderer_end(renderer);
wlr_renderer_bind_buffer(renderer, NULL);

I'm still not very happy with the API, something like this would be better:

wlr_renderer_begin(renderer, buffer);

// Perform the rendering operations

wlr_renderer_end(renderer);

However this would be a breaking change, because compositors call wlr_renderer_begin. Also need to figure out if/how compositors should acquire the buffer.