prefs: Move some DebugOptions to Preferences and clean up

- Move options configuring antialiasing and WebRender shader precache to
  the `Preferences` to group them with other related WebRender and DOM
  settings.
- Rename offscreen canvas preference to make it match more closely to the
  other canvas preferences.
- Disable text antialiasing for canvas during WPT runs to make it
  consistent with the setting for other text.

Signed-off-by: Martin Robinson <[email protected]>

tools/wptrunner/wptrunner/executors/executorservo.py

@@ -231,7 +231,7 @@ def screenshot(self, test, viewport_size, dpi, page_ranges):
             extra_args = ["--exit",
                           "--output=%s" % output_path,
                           "--window-size", viewport_size or "800x600"]
-            debug_opts = "disable-text-aa,load-webfonts-synchronously,replace-surrogates"
+            debug_opts = "load-webfonts-synchronously,replace-surrogates"
 
             if dpi:
                 extra_args += ["--device-pixel-ratio", dpi]