Add web engine screenshot (scuba) tests

[ditman]

Each separate commit in this PR shows the changes needed per test file to get it to run, after bringing all the raw source from our internal repo.

Some test output, when running these on my Linux machine:

$ felt test -t test/golden_tests/engine/
...
00:17 +110 ~2: All tests passed!

Skipped

I had to skip a couple of compositing tests, because we can't control the size of the viewport yet, and they rely on the viewport (iframe) to be smaller than what it is right now:

• Cull rect computation fills screen bounds
• Cull rect computation offset overflows paint bounds

Modified goldens

I needed to update the goldens that rendered text, since most of them used the font sans-serif (system dependant) or Roboto (which may not exist on the system).

I fixed all those instances by setting the font family to Arial and updated the goldens, which should be more multi-platform now... We'll see.

Slightly different golden results (untouched):

• misaligned_pixels_in_canvas_test.png (0.16%)
• misaligned_canvas_test.png (0.16%)
• bitmap_canvas_fills_color_when_transformed.png (0.21%)
• bitmap_canvas_fills_paint_when_transformed.png (0.21%)
• paint_bounds_for_draw_circle.png (0.0008%)
• svg_stroke_line.png (0.0023%)
• svg_quad_bezier.png (0.0817%)
• svg_cubic_bezier.png (0.14%)
• svg_arc_1.png (0.0156%)
• svg_arc_4.png (0.0521%)
• svg_arc_6.png (0.0523%)
• svg_arc_7.png (0.0173%)
• svg_arc_8.png (0.0458%)
• compositing_shifted_clip_rrect.png (0.016%)
• compositing_shifted_physical_shape_clip.png (0.004%)
• compositing_3d_rotate1.png (0.116%)