Expose JointStyle and EndCapStyle on IPen

[antonfirsov]

Modify DrawPathProcessor:

ImageSharp.Drawing/src/ImageSharp.Drawing/Processing/Processors/Drawing/DrawPathProcessor.cs

Line 52 in b58a245

.GenerateOutline(this.Pen.StrokeWidth, this.Pen.StrokePattern);

And probably DrawTextProcessor:

ImageSharp.Drawing/src/ImageSharp.Drawing/Processing/Processors/Text/DrawTextProcessor{TPixel}.cs

Lines 342 to 354 in b58a245

	if (this.renderOutline && this.currentColor == null)
	{
	if (this.Pen.StrokePattern.Length == 0)
	{
	path = path.GenerateOutline(this.Pen.StrokeWidth);
	}
	else
	{
	path = path.GenerateOutline(this.Pen.StrokeWidth, this.Pen.StrokePattern);
	}
	renderData.OutlineMap = this.Render(path);
	}

to use the new properties.

This will simplify life for use cases like #155.

Adding to 1.0 with lowest priority for now, @JimBobSquarePants feel free to kick it out of scope if it becomes blocking.

[antonfirsov]

@jtjackson thanks for your help with this!