CGContext fixes/enhancements

- NEW: Retrieve the underlying Image element from a CPImage with -image.
- NEW: You can now render any arbitrary drawing to a pattern context and use that as a fill or stroke pattern. See CGContextCreatePatternContext, CGContextSetFillPattern and CGContextSetStrokePattern. Works in all canvas-enabled browsers, including IE 9+.
- NEW: An example of using a custom rendered pattern is in Tests/Manual/PatternFillTest.
- NEW: Test if a CPImage is a single image (vs. three/nine part) with -isSingleImage.
- FIXED: With canvas, we have to track ourselves whether the context has a path or not.
- FIXED: All shapes except rects may not be added to a path with no context. If you attempt to do so, an error is logged.
- FIXED: CGPath was not setting the start and current point correctly in some cases.
- FIXED: CGContextAddPath was not moving to the path's start point at the beginning.
- FIXED: Removed superfluous CGContextClosePath commands, fixed some drawing sequences.
- FIXED: Misc. formatting.

Sorry, these changes are canvas only (including IE 9+)! I am not going to spend the time to port these fixes to VML (IE 8).