Celiagg and Image backend have different inital state

[corranwebster]

It looks like the Celiagg backend starts out with a completely transparent image, but the Image/agg backend starts out filled with white. It looks like Quartz and Cairo also start out transparent, QPainter doesn't initialize to anything, so Agg is probably the odd-one out.

Celiagg initializes with zeros:

enable/kiva/celiagg.py

Line 112 in 0ae01f0

buffer = np.zeros(shape, dtype=np.uint8)

Agg initializes with 255:

enable/kiva/agg/__init__.py

Lines 38 to 44 in 0ae01f0

	pixel_map = PixelMap(
	width,
	height,
	pix_format_string_map[pix_format],
	255,
	bool(bottom_up),
	).set_bmp_array()

[corranwebster]

This is causing test failures for the Chaco test suite when run with non-agg backends: enthought/chaco#891

[corranwebster]

Work-around is to explicitly call gc.clear() before drawing anything in tests.