Initial support for sweep gradients in Cairo and CoreGraphics backends

[horasio]

Common code in base.py
Cairo backend uses mesh gradient, so uses fewer curved triangles
CoreGraphics uses flat-shader triangles with acute angle less than half a degree

Edit: this partially addresses #5.

[justvanrossum]

Thanks! This is really cool.

I would love the _buildSweepGradientPatches method to become a function buildSweepGradientPatches in its own module "blackrenderer/backends/sweepGradient.py".

The idea of "blackrenderer/backends/base.py" is that it contains only two things: 1) abstract base classes that define the interfaces for Canvas and Surface and 2) derived functionality that applies to all backend (such as the drawRectXxxx API variants and the simpler transform methods.

Since the sweep gradient support code is not needed for Skia (and may generally depend on the individual backend), I don't think it belongs in base.py.

[justvanrossum]

A more Pythonic way to indicate a default would be None.

Suggested change

	maxAngle=-1,
	maxAngle=None,

[horasio]

ah yes :-)

[justvanrossum]

Suggested change

	if maxAngle == -1:
	if maxAngle is None: