Destroy LayerBuffers before dropping them

This should fix a potential pixmap leak and task failure when the LayerBuffer
is dropped (#1187).
auto merge of #2068 : mbrubeck/servo/1187-buffer-destroy, r=larsbergstrom

This fixes a potential pixmap leak and task failure when the LayerBuffer is dropped (#1187).

I wasn't able to reproduce the failure using the steps from #1187, but based on the stack trace I think this is the correct fix.  Alternately, would it make sense to `impl Drop for LayerBuffer`?