Use memmove instead of memcpy

As the memory could overlap. Found by coverity
[Linux] Clear the buffer before showing the window GOB

or garbage will show up
[Linux] Fix the clipping
[Linux] Cleanup GOB when the window closes

This fixes the crash caused by:

>> load-gui
Fetching GUI...
>> a: view make gob! [text: "hello"]
== make gob! [offset: 1x115 size: 100x100 alpha: 255 text: "Window"]

>> view/as-is/no-wait a
X Error of failed request:  BadWindow (invalid Window parameter)
  Major opcode of failed request:  20 (X_GetProperty)
  Resource id in failed request:  0x1c00002
  Serial number of failed request:  219
  Current serial number in output stream:  219