Stylo: Add support for grid-template-{rows,columns}
This has the implementation of grid's `<track-list>` nightmare.
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stylo: Don't mess up traversal drivers.
Right now it's harmless, but it trips assertions that are nice to have, just in
case it stops being the case.
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Check snapshot id/class flags before looking for those attributes.
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Most elements don't have classes or IDs.
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Not headless by default
Without this, initializing Servo without using the command line (when embedded) requires to explicitly set `opts.headless = false`, which is weird as it's the only option that needs to be changed.
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make gl context current on browser initialization
As explained in servo/webrender#1233, it's necessary to make the current gl context current when the browser is initialized, otherwise the wrong gl context might be used if another window is created before the initialization.
/cc @glennw
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Using TransformStyle::Preserve3D
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The changes were reviewed previously as a part of WR update PRs, but then we decided to move them out.
r? @emilio
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Fix containing block size for absolute root element
The root element doesn't have a containing block, so use the viewport size instead of `self.base.absolute_cb`.
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