egl: forward declare wl_resource for compatibility

Depending on what version of Mesa you have then eglQueryWaylandBuffer
may take a wl_buffer or wl_resource argument and the EGL header will
only forward declare the corresponding type.

The use of wl_buffer has been deprecated and so internally we assume
that eglQueryWaylandBuffer takes a wl_resource but for compatibility we
forward declare wl_resource in case we are building with EGL headers
that still use wl_buffer.

https://bugzilla.gnome.org/show_bug.cgi?id=710926

Reviewed-by: Neil Roberts <neil@linux.intel.com>
Add API to control per-framebuffer depth writing

Add framebuffer methods cogl_framebuffer_[gs]et_depth_write_enabled()
and backend bits to pass the state on to glDepthMask().

This allows us to enable or disable depth writing per-framebuffer, which
if disabled saves us some work in glClear(). When rendering, the flag
is combined with the pipeline's depth writing flag using a logical AND.

Depth writing is enabled by default.

https://bugzilla.gnome.org/show_bug.cgi?id=709827

Reviewed-by: Robert Bragg <robert@linux.intel.com>