Add support for having different GBM formats for different outputs

The gbm-format configuration option can now be specified per-output as
well as in the core config section. If it is not specified it will
default to the format specified in the core section. The
EGL_MESA_configless_context extension is required for this to work. If
this extension is available it will create a context without an
EGLConfig and then it will potentially use a different EGLConfig for
each output.

The gl-renderer interface has been changed so that it takes the EGL
attributes and visual ID in the create_output function as well as in
the create function.
diff --git a/src/compositor-wayland.c b/src/compositor-wayland.c
index 899c329..238946b 100644
--- a/src/compositor-wayland.c
+++ b/src/compositor-wayland.c
@@ -593,7 +593,9 @@
 	}
 
 	if (gl_renderer->output_create(&output->base,
-			output->gl.egl_window) < 0)
+				       output->gl.egl_window,
+				       gl_renderer->alpha_attribs,
+				       NULL) < 0)
 		goto cleanup_window;
 
 	return 0;