Add gradients to the benchmark suite

[rahulporuri]

Add Linear Gradients and Radial Gradients methods in the Benchmark Suite.

[jwiggins]

The graphics context methods linear_gradient and radial_gradient are basically special versions of set_fill_color. A good exercise of these methods would be to create a path with moderate complexity and then fill it will a gradient.

Or if you really want to show how inconsistent the different backends are:

• Set a gradient
• Set the text drawing mode to FILL_STROKE
• Draw some text