For some reason, older WXs on Linux (e.g. Ubuntu 16.04) would
segfault on loading the 4-tile test image. Change one tile to
a circle to fix this (presumably an old and fixed image library
function).
The bitmap pointer is not checked at copy construct. This is
an instant segfault if you copy an empty bitmap.
Fix the constructor and remove the expected test failure, from the
previous commit.
John Beard