Calls into cairo-gles may change the current context, so it was only by
chance that sometimes we had the proper one as current and updated the
correct texture in surface_attach().
In order to fix this, calling display_acquire_window_surface() before
binding the texture for setup is necessary. However this call has the
side effect of allocating a cairo surface for the window. At flush time,
the existence of this surface will cause an eglSwapBuffers(), even
if no rendering was done to it, leading to undefined contents on the
screen. This happens when the idle redraw task runs while there is a
pending frame callback.
Workaround this by moving the texture setup from surface_attach() to the
redraw handler, so that the cairo surface is only allocated when
redering is done.
A wayland compositor doesn't provide a mechanism for buffer sharing between
clients. Under X, one client can render to a Pixmap and another can use it
as a source in a subsequent drawing operations. Wayland doesn't have a
mechanims to share Pixmaps or textures between clients like that, but it's
possible for one client to act as a nested compositor to another client.
This less work than it sounds, since the nested compositor won't have to
provide input devices or even any kind of shell extension. The nested
compositor and its client can be very tightly coupled and have very specific
expectations of what the other process should provide.
In this example, nested.c is a toytoolkit application that uses cairo-gl
for rendering and forks and execs nested-client.c. As it execs the client,
it passes it one end of a socketpair that will be the clients connection
to the nested compositor. The nested compositor doesn't even create a
listening socket.
The client is a minimal GLES2 application, which just renders a spinning
triangle in its frame callback.
