Fix all ASan reported leaks in alpha-blending test

I chose possibly the simplest test from ASan leaks perspective and started fixing all the reported leaks. These are the fixes.

The weston_layer_fini() stuff is not exactly necessary here, I went off an a tangent, because I wanted to write a complete weston_desktop_xwayland_fini() and it needs to fini a layer. But it shouldn't hurt and I guess it's necessary work to avoid use-after-free once we start properly freeing more things. Eventually, one day the screaming to the log should be replaced with simple asserts, but that cannot be done until they do not trigger anymore.