Rendering performance improvements

[VisualMelon]

The rendering performance of rectangle markers is currently poor, as CanvasRenderContext.DrawRectangles does not batch the rectangles. This addresses that by instead passing them off to DrawPolygon which is significantly faster for large numbers of rectangles. The primary goal is to improve performance of markers.

In addition:

• Enables streaming geometry: though this does not have a noticeable performance impact, in theory it puts less load on the property engine
• Replaces ellipses and rectangles with paths with specific geometry: again, this does not noticeably affect performance, but should reduce load on the property engine.
• Fixes a bug where streaming polygons would not be closed
• Fixes dash offsets (noticeable in the example Performance > Line Series, 100k points (dashed line) due to the maximum geometry count)

[arakis]

I can confirm, with this MR, it works MUCH faster, and even Zoom and Panning of simple graphs is more fluent.

[VisualMelon]

This seems to degrade the performance of line series without markers; I'll have to take another look at this.

[VisualMelon]

Closing, largely covered by @BobLd in #38.