tests: add CGContext line attribute tests

[DTW-Thalion]

Adds render tests for the CGContext stroke line attributes: line width, line cap, and line dash. A path is stroked in green on a transparent device-RGB bitmap and the alpha of sample pixels is checked (a pixel is "painted" when its alpha is non-zero).

• Line width - a thin (1pt) line paints along its path but does not reach a pixel three rows away, while a wide (6pt) line does.
• Line cap - a butt cap does not extend past the segment's endpoint, whereas a square cap does.
• Line dash - a [2,2] dash pattern paints its on segments and leaves gaps between them.

Which pixels are painted matches Apple CoreGraphics; the exact antialiased coverage values are not compared.